Video processing method, video processing circuit, liquid crystal display, and electronic apparatus

ABSTRACT

A video processing circuit detects a risk boundary, which a portion of the boundary between a dark pixel and a bright pixel in an image represented by a video signal Vid-in, and is determined by a tilt azimuth of liquid crystal molecules, from the boundary, and corrects a video signal corresponding to at least one of the dark pixel and the bright pixel which is contiguous to the detected risk boundary in at least one field of a plurality of fields constituting one frame such that a period in which the risk boundary is present in one frame period is shortened.

TECHNICAL FIELD

The present invention relates to a technique for reducing displaydefects in a liquid crystal panel.

BACKGROUND ART

A liquid crystal panel has a configuration in which liquid crystal isinterposed between a pair of substrates arranged at a given gap.Specifically, the liquid crystal panel has a configuration in which onesubstrate has pixel electrodes arranged in a matrix to correspond topixels, another substrate has a common electrode in common for thepixels, and liquid crystal is interposed between the pixel electrodesand the common electrode. If a voltage according to a gray-scale levelis applied and held between the pixel electrodes and the commonelectrode, the alignment state of liquid crystal is defined for eachpixel, thereby controlling transmittance or reflectance. Accordingly, inthe above-described configuration, it can be said that, of the electricfield acting on the liquid crystal molecules, only a component in adirection from the pixel electrode toward the common electrode (or theopposite direction), that is, a component in a direction perpendicularto the substrate surface (vertical direction) contributes to displaycontrol.

On the other hand, in recent years, if a pixel pitch is narrowed forreduction in size and higher resolution, an electric field is generatedbetween adjacent pixel electrodes, that is, an electric field in adirection parallel to the substrate surface (lateral direction) isgenerated, and the influence thereof is becoming non-negligible. Forexample, if a lateral electric field is applied to liquid crystal whichshould be driven by a vertical electric field as in the VA (VerticalAlignment) mode, the TN (Twisted Nematic) mode, or the like, analignment defect of liquid crystal (that is, reverse tilt domain)occurs, causing a display defect.

In order to reduce the influence of the reverse tilt domain, a techniquefor devising the structure of a liquid crystal panel by, for example,defining a light shielding layer (opening) according to the shape of apixel electrode (for example, see PTL 1) has been proposed. A techniquefor clipping a video signal having a set value or more on the basis ofthe determination that a reverse tilt domain is generated when anaverage luminance value calculated from a video signal is equal to orsmaller than a threshold value (for example, see PTL 2), a technique forimproving defective image quality by applying a correction voltage to apixel where the phenomenon occurs for defective image quality due to alateral electric field in a matrix driving display device (for example,see PTL 3), or the like has been also proposed.

CITATION LIST Patent Literature

-   PTL 1: JP-A-6-34965 (FIG. 1)-   PTL 2: JP-A-2009-69608 (FIG. 2)-   PTL 3: JP-A-2009-237366 (FIG. 14)

SUMMARY OF INVENTION Technical Problem

However, the technique for reducing the reverse tilt domain with thestructure of the liquid crystal panel described in PTL 1 has suchdrawbacks that the aperture ratio is likely to decrease, and that thetechnique may not be applied to an existing liquid crystal panel whichhas been manufactured without devising the structure. The technique forclipping a video signal having a set value or more described in PTL 2has such a drawback that the brightness of an image to be displayed islimited to the set value. The technique described in PTL 3 requires aprocess procedure in which a potential difference between video signalsinput to two adjacent pixels in the same frame period is detected, whenthere is the potential difference between input video signals to twoadjacent pixels, a correction-target pixel is selected on the basis ofthe potential difference between the two pixels, a scanning direction,and the deposition direction of an alignment film, and a driving voltageis corrected with a correction amount based on the potential differencebetween the two pixels and the potential of the input video signalcorresponding of the correction-target pixel.

Solution to Problem

An advantage of some aspects of the invention is to provide a techniquefor reducing a reverse tilt domain while eliminating these drawbacks.

An aspect of the invention is directed to a video processing methodwhich corrects an input video signal specifying a voltage to be appliedto a liquid crystal element for each pixel and defines the voltage to beapplied to the liquid crystal element on the basis of the correctedvideo signal. The method includes a risk boundary detection step ofdetecting a risk boundary which is a portion of the boundary between afirst pixel whose applied voltage specified by an input video signalfalls below a first voltage and a second pixel whose applied voltageexceeds a second voltage higher than the first voltage, and isdetermined by a tilt azimuth of the liquid crystal, and a correctionstep of correcting a video signal, which specifies a voltage to beapplied to a liquid crystal element corresponding to at least one of thefirst and second pixels contiguous to the risk boundary detected in therisk boundary detection step, in at least one field of a plurality offields constituting one frame such that a period in which the riskboundary is present in one frame period is shortened.

According to the aspect of the invention, the period, in which the riskboundary is present at the same position, in one frame period isshortened, and the stabilization of the defective alignment state of theliquid crystal molecules is suppressed, making it possible to preventthe occurrence of a display defect due to a reverse tilt domain. Sinceit is not necessary to change the structure of the liquid crystal panelhaving the liquid crystal elements, there is no case where the apertureratio decreases, and the invention can be applied to a liquid crystalpanel which has been manufactured without devising the structure. Sincea correction-target pixel is determined under the condition that a pixelis adjacent to the risk boundary, the correction-target pixel is easilyidentified, and the correction value of a video signal to be used can beselected in a wide range.

In the video processing method according to the aspect of the invention,in the correction step, a video signal which specifies a voltage to beapplied to a liquid crystal element corresponding to the first pixelcontiguous to the risk boundary detected in the risk boundary detectionstep or liquid crystal elements corresponding to r (where r is aninteger of 2 or more) continuous first pixels on an opposite side of therisk boundary from the first pixel may be corrected to a video signalwhich specifies the first voltage or higher in any field.

According to this configuration, since the voltage to be applied to theliquid crystal element corresponding to the first pixel is correctedfrom the voltage corresponding to the gray-scale level specified by thevideo signal and is corrected to the first voltage or higher, there isno case where the brightness of an image to be displayed is limited tothe set value.

In the video processing method according to the aspect of the invention,in the correction step, a video signal corresponding to the first pixelas a correction target may be corrected to a video signal of the maximumgray-scale level.

According to this configuration, it is possible to make a user unlikelyto perceive a change in an image to be displayed in accordance with theinput video signal.

In the video processing method according to the aspect of the invention,in the correction step, a video signal which specifies a voltage to beapplied to a liquid crystal element corresponding to the second pixelcontiguous to the risk boundary detected in the risk boundary detectionstep or liquid crystal elements corresponding to s (where s is aninteger of 2 or more) continuous second pixels on an opposite side ofthe risk boundary from the second pixel may be corrected to a videosignal which specifies the second voltage or lower in any field.

According to this configuration, since the voltage to be applied to theliquid crystal element corresponding to the second pixel is correctedfrom the voltage corresponding to the gray-scale level specified by thevideo signal to the second voltage or lower, there is no case where thebrightness of an image to be displayed is limited to the set value.

In the video processing method according to the aspect of the invention,in the correction step, a video signal corresponding to the second pixelas a correction target may be corrected to a video signal of the minimumgray-scale level.

According to this configuration, it is possible to make a user unlikelyto perceive a change in an image to be displayed in accordance with theinput video signal.

In the video processing method according to the aspect of the invention,in the correction step, a video signal which specifies a voltage to beapplied to a liquid crystal element corresponding to the first pixelcontiguous to the risk boundary detected in the risk boundary detectionstep or liquid crystal elements corresponding to r (where r is aninteger of 2 or more) continuous first pixels on an opposite side of therisk boundary from the first pixel may be corrected to a video signalwhich specifies the first voltage or higher in any field, and a videosignal which specifies a voltage to be applied to a liquid crystalelement corresponding to the second pixel contiguous to the detectedrisk boundary or liquid crystal elements corresponding to s (where s isan integer of 2 or more) continuous second pixels on an opposite side ofthe risk boundary from the second pixel may be corrected to a videosignal which specifies the second voltage or lower in the field.

According to this configuration, the effect of suppressing a reversetilt domain increases compared to a case when one of a bright pixel anda dark pixel is a correction target.

The video processing method according to the aspect of the invention mayfurther include a movement detection step of detecting a boundary, whichchanges from a previous frame one frame before a current frame to thecurrent frame, from among the boundaries between the first pixel and thesecond pixel. In the correction step, the video signal corresponding toa correction-target pixel determined by the risk boundary detected inthe risk boundary detection step in the boundary detected in themovement detection step may be corrected.

According to this configuration, it is possible to correct a videosignal focusing on a place where a reverse tilt domain is more likely tooccur.

In the aspect of the invention, in the correction step, the video signalcorresponding to a correction-target pixel determined by the riskboundary moved pixel by pixel from a previous frame to a current framein the boundary detected in the movement detection step may becorrected.

According to this configuration, it is possible to suppress a change inthe input video signal by correcting the video signal focusing on aplace where it is susceptible to a reverse tilt domain and tailingphenomenon is noticeable.

In the aspect of the invention, in the correction step, the video signalcorresponding to a correction-target pixel may not be corrected in anyfield of a plurality of fields.

According to this configuration, it is possible to suppress a change inan image due to the correction of the video signal.

In the aspect of the invention, in the correction step, the video signalcorresponding to a correction-target pixel may be corrected for each ofa plurality of fields.

According to this configuration, even when the video signal is correctedin all of the fields constituting one frame, it is possible to suppressa reverse tilt domain.

Other aspects of the invention can be conceptualized as, in addition tothe video processing method, a video processing circuit, a liquidcrystal display, and an electronic apparatus including the liquidcrystal display.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a liquid crystal display to which a videoprocessing circuit according to a first embodiment of the invention isapplied.

FIG. 2 is a diagram showing an equivalent circuit of a liquid crystalelement in the liquid crystal display.

FIG. 3 is a diagram showing the configuration of the video processingcircuit.

FIG. 4 illustrates diagrams showing the V-T characteristic of a liquidcrystal panel constituting the liquid crystal display.

FIG. 5 illustrates diagrams showing a display operation in the liquidcrystal panel.

FIG. 6 illustrates explanatory views of initial alignment when a VA modeis used in the liquid crystal panel.

FIG. 7 illustrates diagrams illustrating movement of an image in theliquid crystal panel.

FIG. 8 illustrates explanatory views of a reverse tilt which occurs inthe liquid crystal panel.

FIG. 9 illustrates diagrams illustrating movement of an image in theliquid crystal panel.

FIG. 10 illustrates explanatory views of reverse tilt which occurs inthe liquid crystal panel.

FIG. 11 illustrates explanatory views of the outline of a correctionprocess of the video processing circuit.

FIG. 12 illustrates explanatory views of a risk boundary detectionprocedure in the video processing circuit.

FIG. 13 illustrates diagrams showing a correction process in the videoprocessing circuit.

FIG. 14 illustrates diagrams showing when another tilt azimuth angle isset in the liquid crystal panel.

FIG. 15 illustrates diagrams showing when another tilt azimuth angle isset in the liquid crystal panel.

FIG. 16 is an explanatory view of the outline of a correction process ofa video processing circuit according to a second embodiment of theinvention.

FIG. 17 illustrates explanatory views of the outline of a correctionprocess of a video processing circuit according to a third embodiment ofthe invention.

FIG. 18 illustrates diagrams showing a correction process in the videoprocessing circuit.

FIG. 19 illustrates explanatory views of the outline of a correctionprocess of a video processing circuit according to a fourth embodimentof the invention.

FIG. 20 illustrates diagrams showing a correction process in the videoprocessing circuit.

FIG. 21 illustrates diagrams showing the configuration of a videoprocessing circuit according to a fifth embodiment of the invention.

FIG. 22 illustrates diagrams showing a correction process in the videoprocessing circuit.

FIG. 23 illustrates explanatory views of the outline of a correctionprocess of a video processing circuit according to a sixth embodiment ofthe invention.

FIG. 24 illustrates diagrams showing a correction process in the videoprocessing circuit.

FIG. 25 is a diagram showing the configuration of a video processingcircuit according to a seventh embodiment of the invention.

FIG. 26 illustrates explanatory views of a risk boundary detectionprocedure in the video processing circuit.

FIG. 27 illustrates diagrams showing a correction process in the videoprocessing circuit.

FIG. 28 illustrates diagrams showing a correction process in a videoprocessing circuit according to a modification of the invention.

FIG. 29 illustrates diagrams showing a correction process in the videoprocessing circuit of the modification.

FIG. 30 illustrates diagrams showing a correction process in the videoprocessing circuit of the modification.

FIG. 31 illustrates explanatory views of initial alignment when a TNmode is used in the liquid crystal panel.

FIG. 32 illustrates explanatory views of reverse tilt which occurs inthe liquid crystal panel.

FIG. 33 illustrates explanatory views of reverse tilt which occurs inthe liquid crystal panel.

FIG. 34 is a diagram showing a projector to which a liquid crystaldisplay is applied.

FIG. 35 is a diagram showing a display defect or the like by theinfluence of a lateral electric field.

FIG. 36 illustrates explanatory views of the relationship betweeninput/output video signals in usual four-fold speed driving.

DESCRIPTION OF EMBODIMENTS

First Embodiment

Hereinafter, embodiments of the invention will be described withreference to the drawings.

First, a first embodiment of the invention will be described.

FIG. 1 is a block diagram showing the overall configuration of a liquidcrystal display 1 to which a video processing circuit of this embodimentis applied.

As shown in FIG. 1, the liquid crystal display 1 includes a controlcircuit 10, a liquid crystal panel 100, a scanning line driving circuit130, and a data line driving circuit 140. A video signal Vid-in issupplied from a higher-level device to the control circuit 10 insynchronization with a synchronization signal Sync. The video signalVid-in is digital data which specifies the gray-scale level of eachpixel in the liquid crystal panel 100 and supplied in a scanning orderaccording to a vertical scanning signal, a horizontal scanning signal,and a dot clock signal (none of them are shown) included in thesynchronization signal Sync. In this embodiment, the frequency at whichthe video signal Vid-in is supplied is 60 Hz, and the video signalVid-in for displaying an image of one frame (one unit) in the period16.67 milliseconds which is the reciprocal of 60 Hz is supplied.

Although the video signal Vid-in specifies a gray-scale level, it can besafely said that the video signal Vid-in specifies a voltage to beapplied to a liquid crystal element since the voltage to be applied to aliquid crystal element is determined depending on a gray-scale level.

The control circuit 10 includes a scanning control circuit 20 and avideo processing circuit 30. The scanning control circuit 20 generatesvarious control signals and controls the respective units insynchronization with the synchronization signal Sync. The videoprocessing circuit 30, which will be described below in detail,processes the digital video signal Vid-in and outputs an analog datasignal Vx.

The liquid crystal panel 100 has a configuration in which an elementsubstrate (first substrate) 100 a and a counter substrate (secondsubstrate) 100 b are bonded together at a given gap, and liquid crystal105 which is driven by an electric field in the vertical direction isinterposed in the gap. On a surface of the element substrate 100 afacing the counter substrate 100 b, a plurality of m rows of scanninglines 112 are provided along the X (horizontal) direction in thedrawing, and a plurality of n columns of data lines 114 are providedalong the Y (vertical) direction so as to maintain electrical insulationfrom the scanning lines 112.

In this embodiment, for distinguishing between the scanning lines 112,the scanning lines 112 may be referred to as the first, second, third, .. . , (m−1)th, and m-th rows in this order from the top in the drawing.Similarly, for distinguishing between the data lines 114, the data lines114 may be referred to as the first, second, third, . . . , (n−1)th, andn-th columns in this order from the left in the drawing.

A set of an n-channel TFT 116 and a transparent pixel electrode 118having a rectangular shape is provided on the element substrate 100 a soas to correspond to each intersection of the scanning lines 112 and thedata lines 114. The TFT 116 has a gate electrode connected to thescanning line 112, a source electrode connected to the data line 114,and a drain electrode connected to the pixel electrode 118. On a surfaceof the counter substrate 100 b facing the element substrate 100 a, atransparent common electrode 108 is provided over the entire surface. Avoltage LCcom is applied to the common electrode 108 by a circuit (notshown).

In FIG. 1, although the facing surface of the element substrate 100 a ison the rear side of the paper, such that the scanning lines 112, thedata lines 114, the TFTs 116, and the pixel electrodes 118 provided onthe facing surface should be indicated by broken lines, all of them areindicated by solid lines to make the drawing easier to read.

FIG. 2 is a diagram showing an equivalent circuit in the liquid crystalpanel 100.

As shown in FIG. 2, the liquid crystal panel 100 has a configuration inwhich liquid crystal elements 120 are arranged to correspond to theintersections of the scanning lines 112 and the data lines 114 with theliquid crystal 105 interposed between the pixel electrodes 118 and thecommon electrode 108. Though not shown in FIG. 1, in the equivalentcircuit of the liquid crystal panel 100, as shown in FIG. 2, anauxiliary capacitor (storage capacitor) 125 is actually provided inparallel with the liquid crystal element 120. The auxiliary capacitor125 has one end connected to the pixel electrode 118, and the other endconnected to a capacitor line 115 in common. The capacitor line 115 isheld at a constant voltage in terms of time.

If the scanning line 112 is at H level, the TFT 116 whose gate electrodeis connected to the scanning line is turned on, and the pixel electrode118 is connected to the data line 114. For this reason, if a data signalof a voltage according to a gray-scale is supplied to the data line 114when the scanning line 112 is at H level, the data signal is applied tothe pixel electrode 118 through the TFT 116 in the on state. If thescanning line 112 is at L level, the TFT 116 is turned off, but thevoltage applied to the pixel electrode 118 is held by the capacitance ofthe liquid crystal element 120 and the auxiliary capacitor 125.

In the liquid crystal element 120, the molecular alignment state of theliquid crystal 105 changes depending on an electric field generated bythe pixel electrode 118 and the common electrode 108. For this reason,the liquid crystal element 120 has transmittance according to theapplied and held voltage if the liquid crystal element 120 is of atransmissive type. In the liquid crystal panel 100, since transmittancechanges between the liquid crystal elements 120, the liquid crystalelement 120 corresponds to a pixel. An arrangement region of the pixelsserves as a display region 101.

In this embodiment, it is assumed that the liquid crystal 105 is VA modeliquid crystal, and that the normally black mode is used in which theliquid crystal element 120 is in a black state when no voltage isapplied.

The scanning line driving circuit 130 supplies scanning signals Y1, Y2,Y3, . . . , and Ym to the scanning lines 112 in the first, second,third, . . . , and m-th rows in accordance with a control signal Yctrfrom the scanning control circuit 20. Specifically, as shown in FIG.5(a), the scanning line driving circuit 130 selects the scanning lines112 in order of the first, second, third, . . . , (m−1)th, and m-th rowsover a frame. The scanning line driving circuit 130 sets a scanningsignal to the selected scanning line to a selection voltage V_(H) (Hlevel), and sets a scanning signal to other scanning lines to anon-selection voltage V_(L) (L level).

The term “frame” used herein means the time necessary for driving theliquid crystal panel 100 to display a unit of image on the liquidcrystal panel 100. In this embodiment, the frequency of the verticalscanning signal which is controlled by the synchronization signal Syncis 240 Hz. As shown in FIG. 5(a), in the liquid crystal display 1 ofthis embodiment, one frame is divided into four fields of a first fieldto a fourth field, and so-called four-fold speed driving is realized inwhich the scanning lines in the first to m-th rows are scanned in eachfield. That is, on the basis of the video signal Vid-in supplied fromthe higher-level device at a supply speed of 60 Hz, and the liquidcrystal display 1 drives the liquid crystal panel 100 at a driving speedof 240 Hz to display a unit of image on the basis of the video signalVid-in. One field period corresponds to a ¼ frame period, and in thiscase, about 4.16 milliseconds. In the liquid crystal display 1, positivewriting is specified in the first and third fields and negative writingis specified in the second and fourth fields. Thus, the writing polarityis inverted in each field to write data to the pixels. With the use ofmany-fold speed driving, it is possible to reduce impression of residualimage compared to one-fold speed driving.

The data line driving circuit 140 samples the data signal Vx suppliedfrom the video processing circuit 30 for the data lines 114 in the firstto n-th columns as data signals X1 to Xn in accordance with the controlsignal Xctr by the scanning control circuit 20.

In the embodiment of the invention, with regard to a voltage, a groundpotential (not shown) is the reference of zero voltage, except for thevoltage to be applied to the liquid crystal element 120, unlessotherwise specified. The voltage to be applied to the liquid crystalelement 120 is a potential difference between the voltage LCcom of thecommon electrode 108 and the pixel electrode 118, and thus the appliedvoltage is distinguished from other voltages.

In the normally black mode, the relationship between the voltage to beapplied to the liquid crystal element 120 and transmittance isrepresented by the V-T characteristic shown in FIG. 4(a). For thisreason, in order to make the liquid crystal element 120 havetransmittance according to the gray-scale level specified by the videosignal Vid-in, it should suffice that a voltage according to thegray-scale level is applied to the liquid crystal element 120. However,when the voltage to be applied to the liquid crystal element 120 issimply defined in accordance with the gray-scale level specified by thevideo signal Vid-in, a display defect due to a reverse tilt domain mayoccur.

An example of a display defect due to a reverse tilt domain will bedescribed. For example, as shown in FIG. 35, in an image represented bythe video signal Vid-in, when a black pattern having continuous blackpixels on a background having white pixels moves pixel by pixel in theright direction, a kind of tailing phenomenon that a pixel which shouldbe changed from a black pixel to a white pixel in the left edge portion(tail edge portion of movement) does not change to a white pixel appearsdue to the occurrence of a reverse tilt domain.

If the response time of the liquid crystal element is shorter than thetime interval (one frame period) at which the display screen is updatedwhen a region of black pixels on a background having white pixels movesby two pixels every frame, in the liquid crystal panel 100, the tailingphenomenon does not appear (or is hard to be viewed). The reason isconsidered as follows. That is, it is considered that, when a whitepixel and a black pixel are adjacent to each other in one frame, areverse tilt domain may occur in the white pixel, but taking intoconsideration movement of an image, pixels where a reverse tilt domainoccurs are discrete and not visually noticeable.

Viewed from another perspective, in FIG. 35, when a white pattern havingcontinuous white pixels on a background having black pixels moves pixelby pixel in the right direction every frame, a pixel which should bechanged from a black pixel to a white pixel in the right edge portion(leading edge of movement) of the white pattern does not change to awhite pixel due to the occurrence of a reverse tilt domain.

In FIG. 35, for convenience of description, a portion of one line near aboundary in an image is illustrated.

One of the reasons of a display defect due to a reverse tilt domain isconsidered as follows: when the liquid crystal molecules interposed inthe liquid crystal element 120 are unstable, the alignment of the liquidcrystal molecules is disturbed by the influence of a lateral electricfield; and thereafter, the liquid crystal molecules are hard to go intoan alignment state according to the applied voltage.

Here, the condition under which the liquid crystal molecules areaffected by the lateral electric field is a case where the potentialdifference between adjacent pixel electrodes is great, which means acase where a dark pixel at black level (or close to black level) and abright pixel at white level (or close to white level) are adjacent toeach other in an image to be displayed.

It is defined that the dark pixel means a pixel of the liquid crystalelement 120 whose applied voltage is within a voltage range A equal toor higher than a voltage Vbk at black level and lower than a thresholdvalue Vth1 (first voltage) in the normally black mode. For convenience,a transmittance range (gray-scale level) of the liquid crystal elementwhose applied voltage is within the voltage range A is defined as “a”.

It is defined that the bright pixel means a pixel of the liquid crystalelement 120 whose applied voltage is within a voltage range B equal toor higher than a threshold value Vth2 (second voltage) and equal to orlower than a voltage Vwt at white level in the normally black mode. Forconvenience, a transmittance range (gray-scale range) of the liquidcrystal element whose applied voltage is within the voltage range B isdefined as “b”.

When the condition under which the liquid crystal molecules areunstable, the voltage to be applied to the liquid crystal element fallsbelow Vc1 (third voltage) within the voltage range A. When the voltageto be applied to the liquid crystal element falls below Vc1, ananchoring force of a vertical electric field due to the applied voltageis weak compared to an anchoring force due to an alignment film.Accordingly, the alignment state of the liquid crystal molecules islikely to be disturbed by a small external factor. Thereafter, even whenthe applied voltage becomes equal to or higher than Vc1, and the liquidcrystal molecules attempt to tilt in accordance with the appliedvoltage, it takes time for the liquid crystal molecules to respond.Conversely, if the applied voltage is equal to or higher than Vc1, theliquid crystal molecules start to tilt in accordance with the appliedvoltage (transmittance starts to change), and therefore, the alignmentstate of the liquid crystal molecules is stable. For this reason, thevoltage Vc1 is lower than the threshold value Vth1 defined in terms oftransmittance.

When thinking in this way, it can be said that a pixel whose liquidcrystal molecules are unstable before change is in a situation where areverse tilt domain is likely to occur by the influence of a lateralelectric field when a dark pixel and a bright pixel are adjacent to eachother by movement of an image. However, when a study is made in view ofthe initial alignment state of the liquid crystal molecules, a reversetilt domain may occur or may not occur depending on the positionalrelationship between a dark pixel and a bright pixel.

These cases will now be studied below.

FIG. 6(a) is a diagram showing 2-by-2 adjacent pixels in the verticaland horizontal directions in the liquid crystal panel 100. FIG. 6(b) isa simplified sectional view of the liquid crystal panel 100 taken at avertical plane including line p-q in FIG. 6(a).

As shown in FIG. 6, it is assumed that the VA mode liquid crystalmolecules are initially aligned at a tilt angle of THETAa and a tiltazimuth angle of THETAb (=45 degrees) in a state where the potentialdifference (the voltage to be applied to the liquid crystal element)between the pixel electrode 118 and the common electrode 108 is zero. Asdescribed above, since a reverse tilt domain occurs due to the lateralelectric field between the pixel electrodes 118, the behavior of theliquid crystal molecules on the side of the element substrate 100 awhere the pixel electrodes 118 are provided causes a problem. For thisreason, the tilt azimuth angle and the tilt angle of the liquid crystalmolecules are defined based on the side of the pixel electrode 118(element substrate 100 a).

Specifically, as shown in FIG. 6(b), the tilt angle THETAa is defined asan angle made by a long axis Sa of a liquid crystal molecule based on asubstrate normal line Sv when, with one end of the long axis Sa on thepixel electrode 118 side as a fixed point, the other end of the longaxis Sa on the common electrode 108 side tilts.

On the other hand, the tilt azimuth angle THETAb is defined as an anglemade by a substrate vertical plane (vertical plan including the linep-q) including the long axis Sa of the liquid crystal molecule and thesubstrate normal line Sv based on a substrate vertical plane along the Ydirection as the arrangement direction of the data line 114. As viewedin plan from pixel electrode 118 toward the common electrode 108, thetilt azimuth angle THETAb is an angle defined in a clockwise fashionfrom the upper direction of the screen (the opposite direction to the Ydirection) to a direction (upper right direction in FIG. 6(a)) towardthe other end of the long axis of the liquid crystal molecule as astarting point.

Similarly, as viewed in plan from the pixel electrode 118, a directionfrom one end of the liquid crystal molecule on the pixel electrode sidetoward the other end is referred to as a downstream side of the tiltazimuth for convenience, whereas a direction (lower left direction inFIG. 6(a)) from the other end toward one end is referred to as anupstream side of the tilt azimuth for convenience.

In the liquid crystal panel 100 using the liquid crystal 105 with suchan initial alignment, for example, as shown in FIG. 7(a), attention isfocused on 2-by-2=4 pixels surrounded by a broken line. FIG. 7(a) showsa case where a pattern having pixels at black level (black pixels) on abackground of a region having pixels at white level (white pixels) movesin the upper right direction pixel by pixel every frame. In thefollowing description, a frame before t frames (t is a natural number)from the n-th frame is represented by the “(n−t)th frame”, and a frameafter t frames from the n-th frame is represented by the “(n+t)thframe”.

As shown in FIG. 8(a), it is assumed that, in a state where all 2-by-2=4pixels are black pixels in the (n−1)th frame, only one lower left pixelis changed to a white pixel in the n-th frame. As described above, inthe normally black mode, the applied voltage which is the potentialdifference between the pixel electrode 118 and the common electrode 108is greater in a white pixel than in a black pixel. For this reason, inthe lower left pixel to be changed from black to white, as shown in FIG.8(b), the liquid crystal molecules attempt to tilt in a directionperpendicular to an electric field direction, attempting to change froma state indicated by a solid line to a state indicated by a broken line.

However, the potential difference generated in the gap between the pixelelectrode 118 (Wt) of a white pixel and the pixel electrode 118 (Bk) ofa black pixel is substantially equal to the potential differencegenerated between the pixel electrode 118 (Wt) of a white pixel and thecommon electrode 108, and in addition, the gap between the pixelelectrodes is narrower than the gap between the pixel electrode 118 andthe common electrode 108. Accordingly, when compared in terms ofintensity of electric field, the lateral electric field generated in thegap between the pixel electrode 118 (Wt) and the pixel electrode 118(Bk) is stronger than the vertical electric field generated in the gapbetween the pixel electrode 118 (Wt) and the common electrode 108.

Since the lower left pixel is a black pixel whose liquid crystalmolecules are unstable in the (n−1)th frame, it takes time for theliquid crystal molecules to tilt in accordance with the intensity of thevertical electric field. On the other hand, the lateral electric fieldfrom the adjacent pixel electrode 118 (Bk) is stronger than the verticalelectric field induced by applying the voltage at white level to thepixel electrode 118 (Wt). Accordingly, in the pixel to be changed towhite, as shown in FIG. 8(b), a liquid crystal molecule Rv on the sidenext to a black pixel is brought into a reverse tilt state earlier thanthe other liquid crystal molecules which attempt to tilt in accordancewith the vertical electric field.

The liquid crystal molecules Rv which has been earlier brought into thereverse tilt state adversely affects the movement of other liquidcrystal molecules which attempt to tilt in the substrate horizontaldirection in accordance with the vertical electric field as indicated bythe broken line. For this reason, as shown in FIG. 8(c), a region wherereverse tilt occurs in the pixel which should be changed to white doesnot stay within the gap between the pixel which should be changed towhite and the black pixel, but expands from the gap over a wide range soas to erode the pixel which should be changed to white.

From FIG. 8, it can be said that, in a case where there are black pixelsin the vicinity of an attention pixel to be changed to white, when theblack pixels are next to the attention pixel on the upper right, on theright and on the upper, reverse tilt occurs in the inner peripheralregion along the right edge and upper edge of the attention pixel.

The pattern change shown in FIG. 8(a) occurs not only in the exampleshown in FIG. 7(a), but also in a case where the pattern having blackpixels moves in the right direction pixel by pixel every frame as shownin FIG. 7(b), or in a case where the pattern moves in the upperdirection pixel by pixel every frame as shown in FIG. 7(c). As in thedescription of FIG. 35 where viewed from another perspective, thepattern change also occurs in a case where the pattern having whitepixels on the background of the region having black pixels moves in theupper right, right, or upper direction pixel by pixel every frame.

Next, in the liquid crystal panel 100, as shown in FIG. 9(a), when apattern having black pixels on a background of a region having whitepixels moves in the lower left direction pixel by pixel every frame,attention is focused on 2-by-2=4 pixels surrounded by a broken line.

That is, as shown in FIG. 10(a), it is assumed that, in a state whereall 2-by-2=4 pixels are black pixels in the (n−1)th frame, only oneupper right pixel is changed to a white pixel in the n-th frame.

Also after this change, the lateral electric field stronger than thevertical electric field in the gap between the pixel electrode 118 (Wt)and the common electrode 108 is generated in the gap between the pixelelectrode 118 (Bk) of a black pixel and the pixel electrode 118 (Wt) ofa white pixel. With the lateral electric field, as shown in FIG. 10(b),the liquid crystal molecules Rv in the black pixel and on the side nextto the white pixel change in alignment earlier than other liquid crystalmolecules which attempt to tilt in accordance with the vertical electricfield, and therefore are brought into the reverse tilt state. In theblack pixel, however, the vertical electric field does not change fromthe (n−1)th frame, and therefore, the liquid crystal molecules Rv havelittle influence on other liquid crystal molecules. For this reason, inthe pixel which is not changed from a black pixel, as shown in FIG.10(c), the region where reverse tilt occurs is so narrow as to benegligible compared to the example of FIG. 8(c).

On the other hand, in the upper right pixel which is changed from blackto white from among 2-by-2=4 pixels, the initial alignment direction ofthe liquid crystal molecules is a direction which is less likely to beaffected by the lateral electric field. Therefore, even when thevertical electric field is applied, there are few liquid crystalmolecules which are brought into the reverse tilt state. For thisreason, in the upper right pixel, as the intensity of the verticalelectric field increases, the liquid crystal molecules tilt correctly inthe horizontal direction of the substrate surface as indicated by abroken line in FIG. 10(b). As a result, since the upper right pixel ischanged to an intended white pixel, display quality is unlikely to bedeteriorated.

The pattern change shown in FIG. 10(a) occurs not only in the exampleshown in FIG. 9(a), but also in a case where the pattern having blackpixels moves in the left direction pixel by pixel every frame as shownin FIG. 9(b), or in a case where the pattern moves in the lowerdirection pixel by pixel every frame as shown in FIG. 9(c). As in thedescription of FIG. 35 where viewed from another perspective, thepattern change also occurs in a case where the pattern having whitepixels on the background of the region having black pixels moves in thelower left, left, or lower direction pixel by pixel every frame.

From the description of FIGS. 6 to 10, in the VA mode (normally blackmode) liquid crystal, when an n-th frame is focused, it can be said thatthe next pixel is affected by the reverse tilt domain in the n-th framewith all the following requirements satisfied. That is, in the n-thframe, reverse tilt is likely to occur in the bright pixel in a casewhere:

(1) when an n-th frame is focused, a dark pixel and a bright pixel areadjacent to each other, that is, a pixel whose applied voltage is lowand a pixel whose applied voltage is high are adjacent to each other toincrease the lateral electric field;

(2) in the n-th frame, the bright pixel (applied voltage is high) is onthe lower left or left of, or below the adjacent dark pixel (appliedvoltage is low) corresponding to the upstream side of the tilt azimuthin a liquid crystal molecule; and

(3) in a pixel to be changed to the bright pixel in the n-th frame, theliquid crystal molecules are unstable in the (n−1)th frame one framebefore the n-th frame.

Though the reason has already been described, in the requirement (2),when a boundary which represents a portion where a dark pixel and abright pixel are adjacent to each other moves by one pixel from theprevious frame, it is considered that the next pixel is more likely tobe affected by the reverse tilt domain.

FIG. 7 illustrates the example where 2-by-2=4 pixels are black in the(n−1)th frame, and only the lower left pixel is changed to a white pixelin the next n-th frame. In general, however, similar movement isinvolved not only in the (n−1)th frame and the n-th frame, but also overa plurality of frames before and after these frames. For this reason, asshown in (a) to (c) in FIG. 7, in the dark pixel (pixel marked with awhite dot) whose liquid crystal molecules are unstable in the (n−1)thframe, it is considered from the movement of the image pattern thatthere are many cases where a bright pixel is next to the lower left orleft of, or below the dark pixel.

For this reason, in the (n−1)th frame in advance, when a dark pixel anda bright pixel are adjacent to each other in an image represented by thevideo signal Vid-in, and the dark pixel is positioned on the upper rightor right of, or above the bright pixel, a voltage is applied to a liquidcrystal element corresponding to the dark pixel in the n-th frame suchthat a period in which the dark pixel is adjacent to the bright pixelbecomes shorter than one frame period. When this happens, the period inwhich the requirements (1) to (3) are satisfied in the n-th frame aresatisfied is shortened, such that a defective alignment state of theliquid crystal molecules is hard to be generated, and the reverse tiltdomain is not generated in the n-th frame. Specifically, when theapplied voltage specified by the video signal Vid-in falls below Vth1,and when the applied voltage is corrected to a voltage equal to orhigher than Vth1 and applied to the liquid crystal element, the darkpixel is not a dark pixel, and therefore, the risk boundary is notpresent at the same position over one frame period. In this embodiment,it is assumed that, for a dark pixel as a correction target, a videosignal is corrected to a video signal whose gray-scale level is themaximum gray-scale level Cmax such that the lateral electric fieldgenerated between the dark pixel and the adjacent bright pixel can beweakened. The video signal at the maximum gray-scale level Cmaxspecifies the voltage to be applied to a liquid crystal element 120where the potential difference from the voltage LCcom of the commonelectrode 108 becomes 5.0 V, and the applied voltage is the maximumvalue of a voltage which is used for gray-scale expression in the liquidcrystal display 1.

On the basis of the consideration described above, a circuit whichprocesses the video signal Vid-in in the current frame to prevent theoccurrence of a reverse tilt domain in the liquid crystal panel 100 isthe video processing circuit 30 in FIG. 1.

Next, the details of the video processing circuit 30 will be describedwith reference to FIG. 3. As shown in FIG. 3, the video processingcircuit 30 includes a delay circuit 302, a boundary detection unit 304,a correction unit 306, and a D/A converter 308.

The delay circuit 302 includes a FIFO (First In First Out) memory, amultistage latch circuit, or the like. The delay circuit 302 accumulatesthe video signal Vid-in supplied from the higher-level device, reads thevideo signal after a predetermined time elapses, and outputs the videosignal as a video signal Vid-d. The accumulation and readout in thedelay circuit 302 are controlled by the scanning control circuit 20.

The boundary detection unit 304 includes a first detection unit 3041, asecond detection unit 3042, and a determination unit 3043.

The first detection unit 3041 analyzes an image represented by the videosignal Vid-in, and determines whether or not there is a portion where adark pixel (first pixel) in a gray-scale range a and a bright pixel(second pixel) in a gray-scale range b are adjacent to each other in thevertical or horizontal direction. When it is determined that there is anadjacent portion, the first detection unit 3041 detects the adjacentportion as a boundary, and outputs positional information of theboundary.

The term “boundary” used herein thoroughly means a portion where a darkpixel in the gray-scale range a and a bright pixel in the gray-scalerange b are adjacent to each other, that is, a portion where a stronglateral electric field is generated. For this reason, for example, aportion where a pixel in the gray-scale range a and a pixel in agray-scale range d (see FIG. 4(a)) different from the gray-scale range aand the gray-scale range b are adjacent to each other, or a portionwhere a pixel in the gray-scale range b and a pixel in the gray-scalerange d are adjacent to each other is not regarded as a boundary.

The second detection unit 3042 extracts a portion where a dark pixel ison the upper and a bright pixel is on the lower and a portion where adark pixel is on the right and a bright pixel is on the left in theboundary detected by the first detection unit 3041, detects the portionsas a risk boundary, and outputs positional information of the riskboundary.

The determination unit 3043 determines whether or not a pixelrepresented by the video signal Vid-d which is output in a delayedmanner is a dark pixel which is contiguous to the risk boundaryextracted by the second detection unit 3042. When the determinationresult is “Yes”, the determination unit 3043 outputs an output signalwith a flag Q of “1” in a period corresponding to the first and secondfields for the dark pixel. When the determination result is “No”, or ina period corresponding to the third and fourth fields when thedetermination result is “Yes”, the determination unit 3043 outputs theoutput signal with the flag Q of “0”.

A case where “a pixel is contiguous to a risk boundary” includes a casewhere a pixel is adjacent to a risk boundary along one side of thepixel, or a case where a risk boundary which is continuous verticallyand horizontally is at one corner of the pixel. Unless a plurality ofrows (at least three rows) of video signals have been accumulated, thefirst detection unit 3041 may not detect a boundary over in the verticalor horizontal direction in an image to be displayed. The same is alsoapplied to the second detection unit 3042. For this reason, the delaycircuit 302 is provided to adjust a supply timing of the video signalVid-in from the higher-level device.

Since a timing of the video signal Vid-in supplied from the upper-leveldevice differs from a timing of the video signal Vid-d supplied from thedelay circuit 302, their horizontal scanning periods and the like do notcoincide with each other in a precise sense. However, the followingdescription will be provided without specifically distinguishing betweenthem.

The accumulation and the like of the video signal Vid-in in the firstdetection unit 3041 and the second detection unit 3042 are controlled bythe scanning control circuit 20.

As described above, the boundary detection unit 304 performs a riskboundary detection step of detecting a risk boundary.

When the flag Q supplied from the determination unit 3043 is “1”, thecorrection unit 306 corrects the video signal Vid-d of the dark pixel tothe video signal at the maximum gray-scale level Cmax, and outputs thecorrected video signal as a video signal Vid-out. Thus, in the videosignal Vid-out corrected by the correction unit 306, a period in whichthe risk boundary which is contiguous to the dark pixel in one frameperiod is shorter than the video signal Vid-in. In other words, a periodin which there is a pixel which is contiguous to a risk boundary in oneframe period becomes discontinuous. With the correction process in thecorrection unit 306, the risk boundary is not present continuously atthe same position in one frame period. When the flag Q is “0”, thecorrection unit 306 does not correct the video signal and directlyoutputs the video signal Vid-d as the video signal Vid-out (correctionstep).

The D/A converter 308 converts the video signal Vid-out as digital datato an analog data signal Vx. In this embodiment, since the frameinversion scheme is used, the polarity of the data signal Vx is switchedevery rewriting for a unit of image in the liquid crystal panel 100.

Next, the display operation of the liquid crystal display 1 will bedescribed. The video signals Vid-in are supplied from the higher-leveldevice in order of the first row, first column to first row, n-thcolumn, the second row, first column to the second row, n-th column, thethird row, the first column to the third row, n-th column, . . . , them-th row, first column to the m-th row, n-th column over a frame. Thevideo processing circuit 30 performs, a process, such as delay orcorrection, on the video signal Vid-in, and outputs the processed videosignal as the video signal Vid-out.

In view of an effective horizontal scanning period (Ha) in which thevideo signals Vid-out for the first row, first column to the first row,n-th column are output, as shown in FIG. 5(b), the processed videosignal Vid-out is converted to the positive or negative data signal Vxby the D/A converter 308 such that the writing polarity is switchedevery frame in accordance with whether a field is odd-numbered orevennumbered. In the first field, the video signal Vid-out is convertedto the positive data signal. The data signal Vx is sampled for the datalines 114 in the first to n-th columns as data signals X1 to Xn by thedata line driving circuit 140.

In a horizontal scanning period in which the video signals Vid-out forthe first row, first column to the first row, n-th column are output,the scanning control circuit 20 controls the scanning line drivingcircuit 130 such that only the scanning signal Y1 is at H level. If thescanning signal Y1 is at H level, the TFTs 116 in the first row areturned on, such that the data signal sampled for the data line 114 isapplied to the pixel electrodes 118 through the TFTs 116 in the onstate. Thus, a positive voltage according to a gray-scale levelspecified by the video signal Vid-out is written to each of the liquidcrystal elements in the first row, first column to the first row, n-thcolumn.

Subsequently, the video signals Vid-in for the second row, first columnto the second row, n-th column are processed similarly by the videoprocessing circuit 30 and output as the video signal Vid-out. The videosignal is converted to a positive data signal by the D/A converter 308and then sampled for the data lines 114 in the first to n-th columns bythe data line driving circuit 140.

In a horizontal scanning period in which the video signals Vid-out forthe second row, first column to the second row, n-th column are output,only the scanning signal Y2 is at H level by the scanning line drivingcircuit 130, such that the data signals sampled for the data lines 114are applied to the pixel electrodes 118 through the TFTs 116 in thesecond row in the on state. Thus, a positive voltage according to agray-scale level specified by the video signal Vid-out is written toeach of the liquid crystal elements in the second row, first column tothe second row, n-th column.

Thereafter, similar writing operation is performed on the third, fourth,. . . , and m-th rows. Thus, a voltage according to a gray-scale levelspecified by the video signal Vid-out is written to each of the liquidcrystal elements, such that a transmissive image defined by the videosignal Vid-in is produced.

In the next field, similar writing operation is performed except thatthe video signal Vid-out is converted to a negative data signal due tothe polarity inversion of the data signal.

FIG. 5(b) is a voltage waveform diagram showing an example of the datasignal Vx in the first and second fields when the video signals Vid-outfor the first row, first column to the first row, n-th column are outputfrom the video processing circuit 30 over the horizontal scanning period(H). In this embodiment, since the normally black mode is used, the datasignal Vx, if positive, becomes a voltage on the high-potential side(indicated by an upward arrow in the drawing) by an amount correspondingto a gray-scale level processed by the video processing circuit 30 withrespect to a reference voltage Vcnt; while the data signal Vx, ifnegative, becomes a voltage on the low-potential side (indicated by adownward arrow in the drawing) by the amount corresponding to thegray-scale level with respect to the reference voltage Vcnt.

Specifically, the voltage of the data signal Vx, if positive, becomes avoltage shifted from the reference voltage Vcnt by an amountcorresponding to the gray-scale level in a range from a voltage Vw(+)corresponding to white to a voltage Vb(+) corresponding to black; whilethe voltage of the data signal Vx, if negative, becomes a voltageshifted from the reference voltage Vcnt by an amount corresponding tothe gray-scale level in a range from a voltage Vw(−) corresponding towhite to a voltage Vb(−) corresponding to black.

The voltage Vw(+) and the voltage Vw(−) are symmetrical about thevoltage Vcnt. The voltage Vb(+) and the voltage Vb(−) are symmetricalabout the voltage Vcnt.

FIG. 5(b) shows the voltage waveform of the data signal Vx, whichdiffers from the voltage (the potential difference between the pixelelectrode 118 and the common electrode 108) to be applied to the liquidcrystal element 120. The vertical scale of the voltage of the datasignal in FIG. 5(b) is enlarged compared to the voltage waveforms of thescanning signals and the like in FIG. 5(a).

A specific example of the correction process in the video processingcircuit 30 will be described.

First, the relationship between the video signal Vid-in (FIG. 36(a)) andthe video signal Vid-out (FIG. 36(b)) in usual four-fold speed drivingwill be described. (a) and b) in FIG. 36 show a pixel group having aplurality of pixels arranged in one column, and each rectanglecorresponds to one pixel. In FIG. 36 or other drawings, a pixel paintedwith black is a dark pixel, and a pixel painted with white is a brightpixel. In FIG. 36(b), the video signals Vid-out corresponding to thevideo signal Vid-in in the respective frames represent the video signalVid-out corresponding to the first, second, third, and fourth fields inorder from the top in the drawing.

As shown in FIG. 36(a), the video signal Vid-in is supplied at a supplyspeed of 60 Hz, and image display is specified such that, as the firstframe, the second frame, and the third frame progresses by the videosignal Vid-in, an image is scrolled from the left toward the right inthe drawing. In this case, when the video signal Vid-out is output, asshown in FIG. 36(b), the risk boundary is fixedly present at the sameposition over one frame period (that is, over 16.67 milliseconds) havingthe first to fourth fields. If the risk boundary is present at the sameposition over a long period, as described above, a defective alignmentstate of the liquid crystal molecules is likely to be stabilized, and areverse tilt domain is likely to occur in an adjacent pixel.

FIG. 11 is a diagram illustrating the outline of the correction processof the correction unit 306 in the video processing circuit 30 of thisembodiment. In this embodiment, when the video signal Vid-in whichdefines the image is supplied (FIG. 11(a)), the video signal Vid-outshown in FIG. 11(b) is corrected. As shown in FIG. 11(b), in thisembodiment, in the first and second fields corresponding to the firsthalf of one frame period, a dark pixel which is contiguous to the riskboundary is replaced with a bright pixel having the maximum gray-scalelevel Cmax. Thus, as indicated by an arrow in the drawing, the riskboundary moves virtually by only one pixel from the original riskboundary toward the right in the drawing over two fields. In the thirdand fourth fields, since such a correction process is not performed,there is no change in the risk boundary. In this case, the period inwhich the risk boundary is present at the same position is about 8.33milliseconds which is half compared to a case where the correction ofthe correction unit 306 is not performed, and a defective alignmentstate of the liquid crystal molecules is hard to be stabilized, therebysuppressing the occurrence of a reverse tilt domain. As described above,in a frame in which an image based on the video signal Vid-out outputfrom the correction unit 306 is displayed, the period in which the riskboundary is present at the same position is suppressed to be about halfof one frame period.

For example, as shown in FIG. 12(1), when the image represented by thevideo signal Vid-in is an image in which a region having black (dark)pixels with the liquid crystal molecules in an unstable state isdisplayed on the background having white (bright) pixels in thegray-scale range b, a boundary detected by the first detection unit 3041is as shown in FIG. 12(2).

Next, as shown in FIG. 12(3), the second detection unit 3042 extracts aportion where a dark pixel is on the upper and a bright pixel is on thelower and a portion where a dark pixel is on the right and a brightpixel is on the left in the boundary detected by the first detectionunit 3041, and defines the portions as a risk boundary.

In this case, as in a dot-hatched portion of FIG. 13(a), for a darkpixel included in a correction range determined by the extracted riskboundary, the correction unit 306 corrects the video signal to the videosignal at the maximum gray-scale level Cmax in a portion (in this case,two fields) of one frame. In the following description, a dot-hatchedpixel means a dark pixel as a correction target.

A black pixel where a risk boundary which is continuous vertically andhorizontally is positioned at one corner thereof is regarded as “beingcontiguous to the risk boundary”. This is to cope with a situation inwhich an image moves by one pixel in the oblique direction. In contrast,in a black pixel where a risk boundary which is fractured onlyvertically or horizontally is positioned at one corner thereof, a riskboundary which is continuous vertically and horizontally is notpositioned, and therefore, it is not considered that the black pixel isadjacent to the risk boundary. This content is a way to think which isused in common to a bright pixel and the same regardless of a tiltazimuth angle or the like, thus description thereof will not be repeatedappropriately.

According to the first embodiment described above, the period in whichthe risk boundary is present at the same position becomes shorter thanone frame period, and thus, before the liquid crystal molecules arealigned to be brought into the reverse tilt state in one pixel, thepixel is not contiguous to the risk boundary. Thus, the stabilization ofa defective alignment state of the liquid crystal molecules issuppressed, and therefore, it becomes possible to prevent the occurrenceof a display defect due to the above-described reverse tilt domain. Adark pixel as a correction target is determined under the condition thata pixel is adjacent to the risk boundary, and therefore, acorrection-target pixel is easily identified. With a configuration inwhich the liquid crystal panel 100 uses four-fold speed driving, thevideo processing circuit 30 determines the presence/absence of a videosignal in terms of fields, and therefore, it is not necessary to providea complex configuration for correcting a video signal in a portion ofone frame period.

In this embodiment, only a process for detecting a boundary betweenpixels and a risk boundary is performed, instead of a unit of imagerepresented by the video signal, and therefore, it is possible tosuppress an increase in size or complexity of the video processingcircuit compared to a configuration in which two or more units of imageare analyzed to detect movement. It also becomes possible to prevent aregion where a reverse tilt domain is likely to occur from beingcontinuous due to movement of a black pixel.

In this embodiment, in an image defined by the video signal Vid-d,pixels where the video signal is corrected include only pixels which arepositioned on the downstream side of the tilt azimuth with respect to adark pixel. For this reason, it is possible to suppress a portion wheredisplay not based on the video signal Vid-d occurs to be small withouttaking into consideration the tilt azimuth angle compared to aconfiguration in which all dark pixels adjacent to a bright pixel arecorrected uniformly.

In this embodiment, a video signal equal to or greater than a set valueis not clipped uniformly, and therefore, there is no case where avoltage range to be not used is provided to adversely affect thecontrast ratio. It is not necessary to change or the like the structureof the liquid crystal panel 100, and therefore, the aperture ratio isnot degraded. It is also possible to apply the invention to a liquidcrystal panel which has been manufactured without devising thestructure.

Other Examples of Tilt Azimuth Angle

In the above-described embodiment, an example has been described wherethe tilt azimuth angle THETAb is 45 degrees in the VA mode. Next, anexample where the tilt azimuth angle THETAb is other than 45 degreeswill be described.

First, as shown in FIG. 14(a), an example where the tilt azimuth angleTHETAb is 225 degrees will be described. In this example, in a statewhere liquid crystal molecules in an object pixel and a peripheral pixelare unstable, when only the object pixel is changed to a bright pixel,as shown in FIG. 14(b), reverse tilt occurs in the inner peripheralregion along the left edge and lower edge of the object pixel. Thisexample is equivalent to a case where the example shown in FIG. 6 inwhich the tilt azimuth angle THETAb is 45 degrees is rotated by 180degrees.

When the tilt azimuth angle THETAb is 225 degrees, the requirement (2)from among the requirements (1) to (3) that a reverse tilt domain occurswhen the tilt azimuth angle THETAb is 45 degrees is revised as follows.That is, the requirement (2) is revised as follows.

(2) In the n-th frame, the bright pixel (applied voltage is high) ispresent on the upper right or right of, or above the adjacent dark pixel(applied voltage is low) corresponding to the upstream side of the tiltazimuth in a liquid crystal molecule.

The requirements (1) and (3) are not changed.

Accordingly, if the tilt azimuth angle THETAb is 225 degrees, when adark pixel and a bright pixel are adjacent to each other in the n-thframe, and the dark pixel is reversely positioned on the lower left orleft of, or below the bright pixel, it is preferable to take measuresfor a liquid crystal element corresponding to the dark pixel such thatthe liquid crystal molecules are not unstable.

To this end, it should suffice that the correction unit 306 of the videoprocessing circuit 30 corrects the video signal on the basis of the riskboundary of a portion where a dark pixel is on the lower and a brightpixel is on the upper and a portion where a dark pixel is on the leftand a bright pixel is on the right in the boundary detected by the firstdetection unit 3041.

When the tilt azimuth angle THETAb is 225 degrees, in the image shown inFIG. 12(1), the gray-scale level of a black pixel which is contiguous toa risk boundary shown in FIG. 13(c) is corrected to the gray-scale levelCmax.

With this configuration, when the tilt azimuth angle THETAb is 225degrees, in the image defined by the video signal Vid-in, the regionhaving black pixels moves by only one pixel in the lower left, left, orlower direction. Therefore, even when there is a portion where a blackpixel is changed to a white pixel, it is possible to shorten the periodin which the black pixel is contiguous to the risk boundary to a portionof one frame period, making it possible to suppress the occurrence of areverse tilt domain.

Next, as shown in FIG. 15(a), an example where the tilt azimuth angleTHETAb is 90 degrees will be described. In this example, in a statewhere liquid crystal molecules in an object pixel and a peripheral pixelare unstable, when only the object pixel is changed to a bright pixel,as shown in FIG. 15(b), reverse tilt occurs intensively in a regionalong the right edge of the object pixel. For this reason, it may beviewed from perspective that a reverse tilt domain also occurs in theupper side close to the right side and the lower side close to the rightside by an amount corresponding to the width in the right side.

Accordingly, when the tilt azimuth angle THETAb is 90 degrees, therequirement (2) from among the requirements (1) to (3) that a reversetilt domain occurs when the tilt azimuth angle THETAb is 45 degrees isrevised as follows. That is, the requirement (2) is revised as follows.

(2) In the n-th frame, the bright pixel (applied voltage is high) ispositioned not only on the left of the adjacent dark pixel (appliedvoltage is low) corresponding to the upstream side of the tilt azimuthof a liquid crystal molecules, but also above or below the adjacent darkpixel affected by an occurrence region on the left.

The requirements (1) and (3) are not changed.

Accordingly, if the tilt azimuth angle THETAb is 90 degrees, when a darkpixel and a bright pixel are adjacent to each other in the n-th frame,and the dark pixel is reversely positioned on the right of, below, orabove the bright pixel, it is preferable to take measures for a liquidcrystal element corresponding to the dark pixel such that the liquidcrystal molecules are not unstable.

To this end, it should suffice that the correction unit 306 of the videoprocessing circuit 30 corrects the video signal on the basis of the riskboundary of a portion where a dark pixel is on the right and a brightpixel is on the left, a portion where a dark pixel is on the upper and abright pixel is on the lower, and a portion where a dark pixel is on thelower and a bright pixel is on the upper in the boundary detected by thefirst detection unit 3041.

When the tilt azimuth angle THETAb is 90 degrees, in the image shown inFIG. 12(1), the gray-scale level of a black pixel which is contiguous toa risk boundary shown in FIG. 13(b) is corrected to the gray-scale levelCmax.

With this configuration, when the tilt azimuth angle THETAb is 90degrees, in the image defined by the video signal Vid-in, the regionhaving black pixels moves by only one pixel in the upper, upper right,right, lower right, or lower direction. Therefore, even when there is aportion where a black pixel is changed to a white pixel, it is possibleto shorten the period in which the black pixel is contiguous to the riskboundary to a portion of one frame period, making it possible tosuppress the occurrence of a reverse tilt domain.

Second Embodiment

Next, a second embodiment of the invention will be described.

In this embodiment, when correcting a video signal of a dark pixelcontiguous to a risk boundary, the correction unit 306 corrects thevideo signal to a video signal at a middle gray-scale level Cmid, not tothe video signal at the maximum gray-scale level Cmax. The video signalof the middle gray-scale level Cmid is a voltage to be applied to theliquid crystal element 120, and in this case, specifies 2.5 V which is amiddle voltage between an applied voltage corresponding to a maximumgray-scale level and an applied voltage corresponding to a minimumgray-scale level. A configuration, such as a dark pixel as a correctiontarget, except for a correction voltage, is the same as in theabove-described first embodiment.

In this embodiment, when the video signal Vid-in which specifies thedisplay image of the liquid crystal panel 100 is supplied (FIG. 16(a)),as shown in FIG. 16(b), the video signal Vid-out is corrected. In FIG.16 or other drawings, a pixel hatched in an oblique lattice shape is apixel whose gray-scale level is a middle gray-scale level Cmid. Althougha broken line shown in a boundary portion adjacent to the middlegray-scale level Cmid indicates that the portion is not relevant to therisk boundary, the portion may be relevant to the risk boundary. Thesame is applied to the following description.

As shown in FIG. 16(b), in this embodiment, in the first and secondfields which are the first half of each frame, the correction unit 306replaces the dark pixel contiguous to the risk boundary with the pixelhaving the middle gray-scale level Cmid. Thus, a risk boundary is notgenerated between a dark pixel and a bright pixel adjacent to each otherover two fields. Therefore, in this case, the period in which the riskboundary is present at the same position becomes about 8.33 millisecondswhich is half compared to a case where the correction of the correctionunit 306 is not performed, and for the same reason as in theabove-described first embodiment, a reverse tilt domain is hard to begenerated. Since a correction-target pixel is corrected to the videosignal at the middle gray-scale level in the middle of the maximumgray-scale level and the minimum gray-scale level, a change in an imagedue to the correction of the correction unit 306 on the original videosignal Vid-in decreases, making it possible to make the user unlikely toperceive the change.

Third Embodiment

Next, a third embodiment of the invention will be described. In thisembodiment, description will be provided assuming that the normallyblack mode is used. The same is applied to the following embodimentsunless otherwise specified. In the following description, the same partsas those in the first and second embodiments are represented by the samereference numerals, and detailed description thereof will not berepeated appropriately. Although in the above-described first and secondembodiments, the video processing circuit 30 corrects the video signalfor only the dark pixel contiguous to the risk boundary, video signalsof two or more continuous dark pixels on the opposite side of the riskboundary from the dark pixel contiguous to the risk boundary arecorrected. Hereinafter, as in the above-described second embodiment, acase will be described where the video signal of the dark pixel iscorrected to the video signal at the middle gray-scale level Cmid. As inthe first embodiment, however, in a case where the video signal iscorrected to the video signal at the maximum gray-scale level Cmax, ifthe correction voltage is changed, the same correction is carried out.

The video processing circuit 30 of this embodiment is different from theconfiguration of the first embodiment in that the determination contentof the determination unit 3043 is different, and the number of darkpixels as a correction target in the correction unit 306 is changed.

The determination unit 3043 determines whether or not the pixelrepresented by the video signal Vid-d is a dark pixel which iscontiguous to the risk boundary extracted by the second detection unit3042. When the determination result is “Yes”, the determination unit3043 outputs the output signal with the flag Q of “1” in the periodcorresponding to the first and second fields of one frame for r (in thisembodiment, r=2) continuous dark pixels in the opposite direction of therisk boundary from the dark pixel. On the other hand, when thedetermination result is “No”, the determination unit 3043 outputs theoutput signal with the flag Q of “0” in the period corresponding to thethird and fourth fields when the determination result is “Yes”.

When the flag Q supplied from the determination unit 3043 is “1”, thecorrection unit 306 corrects the video signal Vid-d of the dark pixel tothe video signal at the middle gray-scale level Cmid, and outputs thecorrected video signal as the video signal Vid-out. Thus, in the videosignal Vid-out corrected by the correction unit 306, the period in whichthere is the risk boundary which is contiguous to the dark pixel in oneframe period becomes shorter than the video signal Vid-in. On the otherhand, when the flag Q is “0”, the correction unit 306 does not correctthe video signal, and directly outputs the video signal Vid-d as thevideo signal Vid-out.

A specific example of the correction process in the video processingcircuit 30 will be described.

In this embodiment, when a video signal Vid-in shown in FIG. 17(a) issupplied, the video signal Vid-in is corrected to a video signal Vid-outshown in FIG. 17(b). As shown in FIG. 17(b), in this embodiment, in thefirst and second fields which are the first half of each frame, twocontinuous dark pixels on the opposite side of the risk boundary fromthe dark pixel contiguous to the risk boundary are replaced with a pixelat a middle gray-scale level Cmid. Thus, a risk boundary is notgenerated between a dark pixel and a bright pixel over two fields, and arisk boundary is not present continuously in terms of time in one frameperiod. Therefore, in this case, a reverse tilt domain is hard to begenerated.

When an image represented by the video signal Vid-in is as shown in FIG.12(1), and as shown in FIG. 12(3), the second detection unit 3042detects the risk boundary, the correction unit 306 corrects the videosignal to the video signal at the middle gray-scale level Cmid in aportion (in this case, two fields) of one frame for a dot-hatched darkpixel in FIG. 18(a).

In the same way to think as in the first embodiment, when THETAb=90degrees, pixels which satisfy the correction condition in the imagerepresented by FIG. 12(1) are as shown in FIG. 18(b). When THETAb=225degrees, pixels which satisfy the correction condition in the imageshown in FIG. 12(2) are as shown in FIG. 18(c).

According to this embodiment, it is possible to make a change in theapplied voltage due to the correction of the video signals of aplurality of pixels contiguous to the risk boundary unnoticeable.According to the configuration of this embodiment, in addition to theabove, the same effects as in the second embodiment are achieved.

Fourth Embodiment

Next, a fourth embodiment of the invention will be described.

In this embodiment, in the configuration of the first embodiment,instead of the dark pixel contiguous to the risk boundary, video signalsof bright pixels in a correction range determined by the risk boundaryare corrected. In this embodiment, the correction unit 306 does notcorrect the video signal of the dark pixel. In this case, since a brightpixel as a correction target is not a bright pixel after correction,there is no case where the risk boundary is present at the same positionover one frame period. In this embodiment, it is assumed that, for abright pixel as a correction target, the video signal is corrected to avideo signal whose gray-scale level is the minimum gray-scale level Cminsuch that the lateral electric field generated between the bright pixeland the adjacent dark pixel can be reduced. The video signal at theminimum gray-scale level Cmin specifies the voltage to be applied to theliquid crystal element 120 such that the potential difference from thevoltage LCcom of the common electrode 108 is 0 V (equipotential), andthe applied voltage is the minimum value of a voltage which is used forgray-scale expression in the liquid crystal display 1. In the followingdescription, the same parts as those in the first embodiment arerepresented by the same reference numerals, and description thereof willnot be repeated appropriately.

The determination unit 3043 determines whether or not the pixelrepresented by the video signal Vid-d is a bright pixel which iscontiguous to the risk boundary extracted by the second detection unit3042. When the determination result is “Yes”, the determination unit3043 outputs the output signal with the flag Q of “1” in the periodcorresponding to the third and fourth fields. On the other hand, whenthe determination result is “No”, the determination unit 3043 outputsthe output signal with the flag Q of “0” in the period corresponding tothe first and second fields when the determination result is “Yes”.

When the flag Q supplied from the determination unit 3043 is “1”, thecorrection unit 306 corrects the video signal Vid-d of the bright pixelto the video signal at the minimum gray-scale level Cmin, and outputsthe corrected video signal as the video signal Vid-out. Thus, in thevideo signal Vid-out corrected by the correction unit 306, as indicatedby an arrow in the drawing, in fact, the risk boundary moves by only onepixel toward the left in the drawing from the original risk boundaryover two fields. On the other hand, in the first and second fields,since such a correction process is not performed, there is no change inthe risk boundary. Therefore, in the video signal Vid-out, the period inwhich the risk boundary contiguous to the bright pixel is present in oneframe becomes shorter than the video signal Vid-in. When the flag Q is“0”, the correction unit 306 does not correct the video signal, anddirectly outputs the video signal Vid-d as the video signal Vid-out.

A specific example of the correction process in the video processingcircuit 30 will be described.

In this embodiment, when a video signal Vid-in which defines an imagehaving a content shown in FIG. 19(a) is supplied, the video signalVid-in is corrected to a video signal Vid-out shown in FIG. 19(b). Asshown in FIG. 19(b), in this embodiment, in the third and fourth fields,a bright pixel contiguous to the risk boundary is replaced with a darkpixel at the minimum gray-scale level Cmin. Thus, a risk boundary is notgenerated between a dark pixel and a bright pixel adjacent to each otherover two fields. Therefore, in this case, the period in which the riskboundary is present at the same position is about 8.33 millisecondswhich is half compared to a case where the correction of the correctionunit 306 is not performed, and a reverse tilt domain is hard to begenerated.

When the image represented by the video signal Vid-in is as shown inFIG. 12(1), and as shown in FIG. 12(3), the second detection unit 3042detects the risk boundary, the correction unit 306 corrects the videosignal to the video signal at the minimum gray-scale level Cmin in aportion (in this case, two fields) of one frame for an obliquely hatchedbright pixel in FIG. 20(a). Therefore, in this case, a reverse tiltdomain is hard to be generated.

In the same way to think as in the first embodiment, when THETAb=90degrees, pixels which satisfy the correction condition in the imageshown in FIG. 12(1) are as shown in FIG. 20(b). When THETAb=225 degrees,pixels which satisfy the correction condition in the image shown in FIG.12(2) are as shown in FIG. 20(c).

In this embodiment, if the correction process is performed in the firstand second fields, the boundary between a bright pixel and a dark pixelin the third and fourth fields of one frame and the boundary between abright pixel and a dark pixel (a pixel subjected to the correctionprocess) in the first and second fields of a frame next to one frame areat the same position. For this reason, the correction process isperformed in the third and fourth fields.

According to the fourth embodiment described above, the period in whichthe risk boundary is present at the same position becomes shorter thanone frame period, and thus, before the liquid crystal molecules arealigned to be brought into the reverse tilt state in one pixel, thepixel is not contiguous to the risk boundary. Therefore, thestabilization of a defective alignment state of the liquid crystalmolecules is suppressed, making it possible to prevent the occurrence ofa display defect due to the above-described reverse tilt domain.

In addition, the same effects as in the above-described first embodimentare achieved.

Fifth Embodiment

Next, a fifth embodiment of the invention will be described. Although inthe above-described fourth embodiment, the video processing circuit 30corrects the video signal for only a bright pixel contiguous to the riskboundary, video signals of two or more continuous bright pixels on theopposite side of the risk boundary from the bright pixel contiguous tothe risk boundary are corrected. Hereinafter, as in the above-describedsecond embodiment, although a case has been described where the videosignal of the bright pixel is corrected to the video signal at themiddle gray-scale level Cmid, as in the third embodiment, in a casewhere the video signal is corrected to the video signal at the minimumgray-scale level Cmin, if the correction voltage is changed, the samecorrection is carried out.

As described above, the video processing circuit 30 of this embodimentis different from the configuration of the fourth embodiment in that thenumber of bright pixels as a correction target in the correction unit306 and the correction voltage are changed.

The determination unit 3043 determines whether or not the pixelrepresented by the video signal Vid-d is a bright pixel contiguous tothe risk boundary extracted by the second detection unit 3042. When thedetermination result is “Yes”, the determination unit 3043 outputs theoutput signal with the flag Q of “1” in the period corresponding to thefirst and second fields for s (in this embodiment, s=2) continuousbright pixels in the opposite direction of the risk boundary from thebright pixel. When the determination result is “No”, the determinationunit 3043 outputs the output signal with the flag Q of “0” in the periodcorresponding to the third and fourth fields when the determinationresult is “Yes”.

When the flag Q supplied from the determination unit 3043 is “1”, thecorrection unit 306 corrects the video signal Vid-d of the bright pixelto the video signal at the middle gray-scale level Cmid, and outputs thecorrected video signal as the video signal Vid-out. Thus, in the videosignal Vid-out corrected by the correction unit 306, the period in whichthe risk boundary contiguous to the bright pixel is present in one frameperiod becomes shorter than the video signal Vid-in. When the flag Q is“0”, the correction unit 306 does not correct the video signal, anddirectly outputs the video signal Vid-d as the video signal Vid-out.

A specific example of the correction process in the video processingcircuit 30 will be described.

In this embodiment, when a video signal Vid-in which defines an imagehaving a content shown in FIG. 21(a) is supplied, the video signalVid-in is corrected to a video signal Vid-out shown in FIG. 21(b). Asshown in FIG. 21(b), in this embodiment, in the first and second fieldswhich are the first half of each frame, two continuous bright pixels onthe opposite side of the risk boundary from a bright pixel contiguous toa risk boundary are replaced with pixels at the middle gray-scale levelCmid. Thus, a boundary is not generated between a dark pixel and abright pixel over two fields. Therefore, in this case, a reverse tiltdomain is hard to be generated.

When the image represented by the video signal Vid-in is as shown inFIG. 12(1), and as shown in FIG. 12(3), the second detection unit 3042detects the risk boundary, the correction unit 306 corrects the videosignal to the video signal at the middle gray-scale level Cmid in aportion (in this case, two fields) of one frame for an obliquely hatchedbright pixel in FIG. 22(a).

In the same way to think as in the first embodiment, when THETAb=90degrees, pixels which satisfy the correction condition in the imageshown in FIG. 12(1) are as shown in FIG. 22(b). When THETAb=225 degrees,pixels which satisfy the correction condition in the image shown in FIG.12(2) are as shown in FIG. 22(c).

According to this embodiment, it is possible to make a change in theapplied voltage due to the correction of the video signals of aplurality of pixels contiguous to the risk boundary unnoticeable.According to the configuration of this embodiment, in addition to theabove, the same effects as in the fourth embodiment are achieved.

Sixth Embodiment

Next, a sixth embodiment of the invention will be described.

In the following description, the same parts as those in the firstembodiment are represented by the same reference numerals, anddescription thereof will not be repeated appropriately. In thisembodiment, both of the correction of the dark pixel described in thefirst to third embodiments and the correction of the bright pixeldescribed in the fourth and fifth embodiments are performed.Hereinafter, description will be provided as to a case where a darkpixel is corrected as in the third embodiment and a bright pixel iscorrected as in the fifth embodiment.

The video processing circuit 30 of this embodiment is different from thevideo processing circuit 30 of the first embodiment in that thedetermination content of the determination unit 3043 is changed, and apixel as a correction target in the correction unit 306 is changed.

The determination unit 3043 performs both types of determinationdescribed in the above-described third and fifth embodiments. That is,the determination unit 3043 outputs the output signal with the flag Q of“1” in the period corresponding to the first and second fields for r (inthis embodiment, r=2) continuous dark pixels in the opposite directionof the risk boundary from a dark pixel contiguous to the risk boundaryextracted by the second detection unit 3042, and also outputs the outputsignal with the flag Q of “1” in the period corresponding to the firstand second fields for s (in this embodiment, s=2) continuous brightpixels in the opposite direction of the risk boundary from a brightpixel contiguous to the risk boundary. On the other hand, thedetermination unit 3043 outputs the output signal with the flag Q of “0”for other pixels.

When the flag Q supplied from the determination unit 3043 is “1”, thecorrection unit 306 corrects a video signal of a bright pixel when theflag Q is “1” to the video signal at the gray-scale level Cmid, andoutputs the corrected video signal as the video signal Vid-out. Thus, inthe video signal Vid-out corrected by the correction unit 306, theperiod in which the risk boundary contiguous to the dark pixel and thebright pixel is present in one frame period becomes shorter than thevideo signal Vid-in. When the flag Q is “0”, the correction unit 306does not correct the video signal, and directly outputs the video signalVid-d as the video signal Vid-out.

A specific example of the correction process in the video processingcircuit 30 will be described.

In this embodiment, when a video signal Vid-in which defines an imageshown in FIG. 23(a) is supplied, the video signal Vid-in is corrected toa video signal Vid-out shown in FIG. 23(b). As shown in FIG. 23(b), inthis embodiment, in the first and second fields which are the first halfof each frame, two continuous bright pixels on the opposite side of therisk boundary from a bright pixel contiguous to the risk boundary arereplaced with pixels at the middle gray-scale level Cmid, and twocontinuous dark pixels on the opposite side of the risk boundary from adark pixel contiguous to the risk boundary are replaced with pixels atthe middle gray-scale level Cmid. Thus, a boundary is not generatedbetween a dark pixel and a bright pixel over two fields. Therefore, inthis case, the period in which the risk boundary is present at the sameposition is 8.33 milliseconds which is half compared to a case where thecorrection of the correction unit 306 is not performed, and a reversetilt domain is hard to be generated.

When the image represented by the video signal Vid-in is as shown inFIG. 12(1), and as shown in FIG. 12(3), the second detection unit 3042detects the risk boundary, the correction unit 306 corrects the videosignal to the video signal at the middle gray-scale level Cmid in aportion (in this case, two fields) of one frame for a dot-hatched darkpixel and an obliquely hatched bright pixel in FIG. 24(a).

In the same way to think as in the first embodiment, when THETAb=90degrees, pixels which satisfy the correction condition in the imageshown in FIG. 12(1) are as shown in FIG. 24(b). When THETAb=225 degrees,pixels which satisfy the correction condition in the image shown in FIG.12(2) are as shown in FIG. 24(c).

According to this embodiment, the same effects as in the third and fifthembodiments are achieved, and the effect of suppressing a reverse tiltdomain increases compared to a case where either a bright pixel or adark pixel is a correction target. The configuration for correcting avideo signal of a dark pixel as in the first and second embodiments andthe configuration for correcting a video signal of a bright pixel as inthe fourth embodiment may be appropriately combined to correct videosignals of both of a dark pixel and a bright pixel.

Seventh Embodiment

Next, a seventh embodiment of the invention will be described.

When movement is involved in an image, in a pixel contiguous to a riskboundary in an image of a current frame represented by a video signalVid-in, if movement including a previous frame one frame before thecurrent frame is taken into consideration, it is necessary to correctthe video signal or it is not necessary to correct the video signal. Inthis embodiment, in a configuration in which video signals of a darkpixel and a bright pixel are corrected as in the sixth embodiment, acorrection-target pixel is determined taking into consideration movementof the image from the previous frame to the current frame. In thefollowing description, the same parts as those in the sixth embodimentare represented by the same reference numerals, and description thereofwill not be repeated appropriately.

Next, the details of the video processing circuit 30 will be describedwith reference to FIG. 25. As shown in FIG. 25, the video processingcircuit 30 includes a delay circuit 302, a boundary detection unit 312,a correction unit 306, and a D/A converter 308.

In this embodiment, the boundary detection unit 312 includes a currentframe detection unit 3121, a delay circuit 3122, a previous framedetection unit 3123, and a determination unit 3124.

The current frame detection unit 3121 analyzes the image represented bythe video signal Vid-in in the current frame, and determines whether ornot there is a portion where a dark pixel (first pixel) in a gray-scalerange a and a bright pixel (second pixel) in a gray-scale range b areadjacent to each other in the vertical or horizontal direction. When itis determined that there is an adjacent portion, the current framedetection unit 3121 detects the adjacent portion as a risk boundary, andoutputs positional information Risk_edge(n) of the risk boundary.

The delay circuit 3122 has the same configuration as the delay circuit302, and delays and outputs the supplied video signal Vid-in by oneframe period.

The previous frame detection unit 3123 analyzes the image represented bythe video signal Vid-in in the previous frame output from the delaycircuit 3122, and determines whether or not there is a portion where adark pixel and a bright pixel are adjacent to each other in the verticalor horizontal direction. When it is determined that there is an adjacentportion, the previous frame detection unit 3123 detects the adjacentportion as a risk boundary, and outputs positional informationRisk_edge(n−1) of the risk boundary.

The determination unit 3124 determines whether or not a pixelrepresented by the video signal Vid-d is a bright pixel or a dark pixelcontiguous to the risk boundary extracted by the previous framedetection unit 3123 on the basis of the positional informationRisk_edge(n) and Risk_edge(n−1) output from the current frame detectionunit 3121 and the previous frame detection unit 3123, and whether or notthe pixel is contiguous to the risk boundary which is changed from theprevious frame to the current frame. That is, the determination unit3124 functions as a movement detection unit which performs a movementdetection step of detecting movement of the image by the risk boundarywhich is changed from the previous frame to the current frame.

When the determination result is “Yes”, the determination unit 3124outputs the output signal with the flag Q of “1” in the periodcorresponding to the first and second fields for r (in this embodiment,r=2) continuous dark pixels in the opposite direction of the riskboundary from the dark pixel, and also outputs the output signal withthe flag Q of “1” in the period corresponding to the first and secondfields for s (in this embodiment, s=2) continuous bright pixels in theopposite direction of the risk boundary from the bright pixel contiguousto the risk boundary. On the other hand, the determination unit 3143outputs the output signal with the flag Q of “0” for other pixels.

When the flag Q supplied from the determination unit 3124 is “1”, thecorrection unit 306 corrects the video signal Vid-d of the pixel to thevideo signal at the middle gray-scale level Cmid, and outputs thecorrected video signal as the video signal Vid-out. Thus, in the videosignal Vid-out corrected by the correction unit 306, the period in whichthe risk boundary contiguous to the dark pixel and the bright pixel ispresent in one frame period becomes shorter than the video signalVid-in. When the flag Q is “0”, the correction unit 306 does not correctthe video signal, and directly outputs the video signal Vid-d as thevideo signal Vid-out.

A specific example of the correction process in the video processingcircuit 30 will be described.

In the video processing circuit 30 of this embodiment, while the riskboundary to be detected is different from the method described in thefirst embodiment, how a video signal of a pixel is corrected on thebasis of the risk boundary is the same as in the above-described sixthembodiment.

When the image represented by the video signal Vid-in in the previousframe is, for example, as shown in FIG. 26(1), and the image representedby the video signal Vid-in in the current frame is, for example, asshown in FIG. 26(2), that is, when a pattern having dark pixels in thegray-scale range a is scrolled in the left direction on a backgroundhaving bright pixels in the gray-scale range b, a boundary detected bythe previous frame detection unit 3123 is as shown in FIG. 26(1), and aboundary detected by the current frame detection unit 3121 is as shownin FIG. 26(2). In the boundary detected by the current frame detectionunit 3121, a portion which does not overlap the boundary detected by theprevious frame detection unit 3123 becomes a boundary which is changedfrom the previous frame to the current frame. Thus, the risk boundary ofthis embodiment is as shown in FIG. 26(3). In the determination unit3124, a pixel adjacent to a portion corresponding to the risk boundaryof a portion where a dark pixel is on the upper and a bright pixel is onthe lower and a portion where a dark pixel is on the right and a brightpixel is on the left in the changed boundary becomes a correctiontarget.

A video signal Vid-out when the image represented by the video signalVid-in is changed from FIG. 26(1) to FIG. 26(2) is shown in FIG. 27(a).The correction unit 306 corrects the video signal to the video signal atthe middle gray-scale level Cmid in a portion (in this case, two fields)of one frame for a dot-hatched dark pixel and an obliquely hatchedbright pixel in FIG. 27(a).

In the same way to think as in the first embodiment, when THETAb=90degrees, pixels which satisfy the correction condition in the imageshown in FIG. 12(1) are as shown in FIG. 27(b). When THETAb=225 degrees,pixels which satisfy the correction condition in the image shown in FIG.24(b) are as shown in FIG. 27(c).

According to the seventh embodiment described above, the functionaleffects common to the above-described sixth embodiment can be achieved,and a video signal can be corrected focusing on a place where a reversetilt domain is more likely to occur. Therefore, it is possible toeffectively suppress the occurrence of a reverse tilt domain whilefurther suppressing variation in the video signal.

Although in this embodiment, the risk boundary has been detected fromthe boundary changed from the previous frame to the current frame, thisconfiguration may be applied to the configuration of the above-describedfirst to sixth embodiments.

Eighth Embodiment

Next, an eighth embodiment of the invention will be described.

In the following description, the same parts as those in the seventhembodiment are represented by the same reference numerals, anddescription thereof will not be repeated appropriately.

In the above-described seventh embodiment, video signals of pixels arecorrected on the basis of a bright pixel and a dark pixel adjacent toeach other with a risk boundary interposed therebetween taking intoconsideration movement of an image. In contrast, in this embodiment, thevideo processing circuit 30 detects a boundary between a dark pixel anda bright pixel adjacent to each other in the current frame, and definespixels contiguous to a risk boundary having moved from the previousframe to the current frame by one pixel (in the vertical, horizontal, oroblique direction) in the detected boundary as a correction target. Asalready described with reference to FIG. 35, when a region having darkpixels on a background having bright pixels moves by two or more pixelsevery frame, such a tailing phenomenon does not appear (or is hard to beviewed). Accordingly, the video processing circuit 30 defines pixelscontiguous to the risk boundary having moved by one pixel as acorrection target, and does not define other pixels as a correctiontarget.

In this embodiment, the determination unit 3124 determines “Yes” foronly the pixels contiguous to the risk boundary having moved by onepixel from the boundary detection results of the current frame detectionunit 3121 and the previous frame detection unit 3123, and determines“No” for pixels contiguous to the risk boundary not having moved fromthe previous frame and the risk boundary having moved by two or morepixels. The functions which are realized by other units of the videoprocessing circuit 30 are the same as in the seventh embodiment. Withthis configuration, as shown in FIG. 35, when an image moves by only onepixel for one frame, the correction unit 306 performs correction so asto suppress a reverse tilt domain. Otherwise, correction is notperformed.

Thus, the correction unit 306 can perform correction further focusing ona place where a reverse tilt domain is more likely to occur. Therefore,it is possible to effectively suppress the occurrence of a reverse tiltdomain while suppressing variation in the video signal.

In the configuration of this embodiment, the same effects as in theabove-described seventh embodiment are achieved. The configuration inwhich a correction-target pixel is determined on the basis of the riskboundary having moved by only one pixel may also be applied to theconfiguration of the above-described first to sixth embodiments.

Modifications

Modification 1

Although in the video processing circuit 30 of each embodiment, videosignals are corrected in the first and second fields (or the third andfourth fields) from among the four fields constituting one frame, videosignals may be corrected in other fields. For example, in an exampleshown in FIG. 28, the video processing circuit 30 corrects video signalsin the first and third fields. With this configuration, since the periodin which the risk boundary is continuous at the same position is onefield (about 4.16 milliseconds) to the maximum, as in theabove-described embodiments, the effect of suppressing a reverse tiltdomain is achieved.

Although in the video processing circuit 30 of each embodiment, videosignals are corrected in two fields from among the four fieldsconstituting one frame, video signals may be corrected using a differentnumber of fields. For example, in an example shown in FIG. 29, the videoprocessing circuit 30 corrects video signals only in the first field,and does not correct video signals in the second to fourth fields. Withthis configuration, it is possible to further suppress changes in videosignals, and to further prevent the user from perceiving changes intransmittance of pixels. Video signals may be corrected in one fieldfrom among the second to fourth fields.

The video processing circuit 30 may correct video signals in all of thefour fields. For example, in an example of FIG. 30, the video processingcircuit 30 corrects video signals in the first and second fields as inthe first embodiment, and corrects video signals in the third and fourthfields as in the fourth embodiment. The correction processes of thefirst to seventh embodiments may be combined appropriately. In short, itshould suffice that the video processing circuit 30 corrects videosignals such that a risk boundary is present continuously at the sameposition in one frame period, and a risk boundary is not present at thesame position over one frame.

Modification 2

In the above-described embodiments, an example has been described wherethe VA mode is used for the liquid crystal 105. Next, an example wherethe TN mode is used for the liquid crystal 105 will be described.

FIG. 31(a) is a diagram showing 2-by-2 pixels in a liquid crystal panel100. FIG. 31(b) is a simplified sectional view taken at a vertical planeincluding line p-q in FIG. 31(a).

As shown in (a) and (b) in FIG. 31, it is assumed that, TN mode liquidcrystal molecules are initially aligned at a tilt angle of THETAa and atilt azimuth angle of THETAb (=45 degrees) in a state where thepotential difference between the pixel electrode 118 and the commonelectrode 108 is zero. In the TN mode, contrary to the VA mode, theliquid crystal molecules tilt in the substrate horizontal direction, andtherefore, the tilt angle THETAa in the TN mode is greater than thevalue of the VA mode.

In an example where the TN mode is used for the liquid crystal 105,there are many cases where the normally white mode in which the liquidcrystal element 120 is in a white state with no application of voltageis used because a high contrast ratio and the like are obtained.

For this reason, when the TN mode is used for the liquid crystal 105 andthe normally white mode is used, the relationship between the voltage tobe applied to the liquid crystal element 120 and transmittance isrepresented by the V-T characteristic shown in FIG. 4(b), in which thetransmittance decreases as the applied voltage increases. However,similarly to the normally black mode, when a pixel belonging to thegray-scale range a and a pixel belonging to the gray-scale range b areadjacent to each other, a risk boundary is generated, and a reverse tiltdomain is generated.

In the normally white mode in the TN mode, as shown in FIG. 32(a), it isassumed that, in a state where all 2-by-2=4 pixels are white pixelswhose liquid crystal molecules are unstable in the (n−1)th frame, onlythe upper right pixel is changed to a black pixel in the n-th frame. Asdescribed above, in the normally white mode, the potential differencebetween the pixel electrode 118 and the common electrode 108 is greaterin a black pixel than in a white pixel, contrary to the normally blackmode. For this reason, in the upper right pixel which is changed fromwhite to black, as shown in FIG. 32(b), liquid crystal molecules attemptto rise in a direction (direction perpendicular to the substratesurface) along the electric field direction, attempting to change from astate indicated by a solid line to a state indicated by a broken line.

However, the potential difference generated in the gap between the pixelelectrode 118 (Wt) of a white pixel and the pixel electrode 118 (Bk) ofa black pixel is substantially the same as the potential differencegenerated between the pixel electrode 118 (Bk) of a black pixel and thecommon electrode 108, and the gap between the pixel electrodes isnarrower than the gap between the pixel electrode 118 and the commonelectrode 108. Therefore, when compared in terms of intensity ofelectric field, the lateral electric field generated in the gap betweenthe pixel electrode 118 (Wt) and the pixel electrode 118 (Bk) isstronger than the vertical electric field generated in the gap betweenthe pixel electrode 118 (Bk) and the common electrode 108.

Since the upper right pixel is a white pixel whose liquid crystalmolecules are unstable in the (n−1)th frame, it takes time for theliquid crystal molecules to tilt in accordance with the intensity of thevertical electric field. On the other hand, the lateral electric fieldfrom the adjacent pixel electrode 118 (Wt) is stronger than the verticalelectric field induced by applying a voltage at black level to the pixelelectrode 118 (Bk). Therefore, in the pixel to be changed to black, asshown in FIG. 31(b), the liquid crystal molecules Rv on the sideadjacent to a white pixel is brought into the reverse tilt state earlierthan other liquid crystal molecules which attempt to tilt in accordancewith the vertical electric field.

The liquid crystal molecules Rv which have been earlier brought into thereverse tilt state adversely affect the movement of other liquid crystalmolecules which attempt to rise in the substrate horizontal direction asindicated by a broken line. For this reason, as shown in FIG. 32(c), aregion where reverse tilt occurs in the pixel which should be changed toblack does not stay within the gap between a pixel which should bechanged to black and a white pixel, and expands from the gap over a widerange so as to erode the pixel which should be changed to black.

Accordingly, from the content shown in FIG. 32, in the case where whitepixels are in the vicinity of an attention pixel to be changed to black,when the white pixels are adjacent to the lower left and left of, andbelow the attention pixel, reverse tilt occurs in the inner peripheralregion along the left edge and lower edge of the attention pixel.

As shown in FIG. 33(a), it is assumed that, in a state where all2-by-2=4 pixels are white pixels whose liquid crystal molecules areunstable in the (n−1)th frame, only one lower left pixel is changed to ablack pixel in the n-th frame. Also in this change, in the gap betweenthe pixel electrode 118 (Bk) of a black pixel and the pixel electrode118 (Wt) of a white pixel, a lateral electric field stronger than avertical electric field in the gap between the pixel electrode 118 (Bk)and the common electrode 108 is generated. With the lateral electricfield, as shown in FIG. 33(b), the liquid crystal molecules Rv in awhite pixel and on the side adjacent to a black pixel change inalignment earlier than other liquid crystal molecules which attempt totilt in accordance with the vertical electric field, and are broughtinto the reverse tilt state. In the white pixel, however, since theintensity of the vertical electric field does not change from the(n−1)th frame, the liquid crystal molecules Rv have little influence onother liquid crystal molecules. For this reason, as shown in FIG. 33(c),the region where reverse tilt occurs in the pixel which is not changedfrom a white pixel is so narrow as to be negligible compared to theexample of FIG. 32(c).

In the lower left pixel which is changed from white to black from amongthe 2-by-2=4 pixels, the initial alignment direction of liquid crystalmolecules is less likely to be affected by the lateral electric field.Therefore, even when the vertical electric field is applied, there arefew liquid crystal molecules which are brought into the reverse tiltstate. For this reason, in the lower left pixel, as the intensity of thevertical electric field increases, the liquid crystal moleculescorrectly rise in the direction perpendicular to the substrate surfaceas indicated by a broken line in FIG. 27(b). As a result, since thelower left pixel is changed to an intended black pixel, display qualityis not deteriorated.

For this reason, in the normally white mode in which the tilt azimuthangle THETAb is 45 degrees in the TN mode, in the n-th frame, reversetilt occurs in the dark pixel under the requirement (1) and in a casewhere:

(2) in the n-th frame, the dark pixel (applied voltage is high) ispositioned on the upper right or right of, or above the adjacent brightpixel (applied voltage is low); and

(3) in a pixel which is changed to the dark pixel in the n-th frame, theliquid crystal molecules are unstable in the (n−1)th frame one framebefore the n-th frame.

Accordingly, when reconsidering the occurrence state based on the(n+1)th frame, it can be said that, even when a dark pixel satisfies theabove-described positional relationship in the (n+1)th frame due tomovement of an image, it is preferable to take measures such that therisk boundary contiguous to the pixel is not present continuously at thesame position in one frame period in the n-th frame before change.

In the normally white mode, contrary to the normally black mode, theconfiguration of the video processing circuit 30 may be changed asfollows taking into consideration that, the higher (brighter) thegray-scale level, the lower the voltage to be applied to the liquidcrystal element.

That is, it should suffice that, in the video processing circuit 30, thesecond detection unit 3042, or the current frame detection unit 3121 andthe previous frame detection unit 3123 which detect the risk boundaryextract a portion where a dark pixel is on the lower and a bright pixelis on the upper and a portion where a dark pixel is on the left and abright pixel is on the right in the n-th frame, and detect the portionsas the risk boundary. A pixel whose video signal is corrected by thecorrection unit 306 on the basis of the risk boundary is as described inthe above-described first to eighth embodiments.

Although in this example, an example has been described where the tiltazimuth angle THETAb is 45 degrees in the TN mode, taking intoconsideration that the occurrence direction of a reverse tilt domain isopposite to the VA mode, measures when the tilt azimuth angle THETAb isother than 45 degrees and the configuration therefor can also beanalogized easily from the above description.

If it is assumed that the movement direction of an image pattern is onlythe horizontal direction, it becomes possible to simplify theconfiguration compared to a case where it is assumed that the movementdirection of the image pattern also includes the vertical direction orthe oblique direction.

Although an example has been described where the tilt azimuth angleTHETAb is 45 degrees in the VA mode, the same is also applied to a casewhere the tilt azimuth angle THETAb is 225 degrees in the VA mode.

Modification 3

Although in the above-described embodiments, r=2 and s=2, these valuesare just an example. Therefore, r and s may be an integer of “2” ormore, and these values may be different from each other.

The video processing circuit according to the invention is not limitedto a liquid crystal display which uses four-fold speed driving, and canbe applied to a liquid crystal display which uses many-fold speeddriving, such as two-fold speed driving or eight-fold speed driving. Inshort, in the embodiment of the invention, it should suffice that, whenan image is displayed on the basis of a video signal, a video signal ofat least one of a bright pixel and a dark pixel is corrected in at leastone of a plurality of fields constituting one frame such that a periodin which a risk boundary is present in one frame period is shortened anda risk boundary is not present continuously at the same position in oneframe period.

When correcting a dark pixel, the correction unit 306 corrects the videosignal to a video signal which specifies a voltage equal to or higherthan the threshold value Vth1 as the voltage to be applied to the liquidcrystal element 120. When correcting a bright pixel, the correction unit306 corrects the video signal to a video signal which specifies avoltage equal to or lower than the threshold value Vth2 as the voltageto be applied to the liquid crystal element 120. In this way, the effectof suppressing a reverse tilt domain is achieved.

Modification 4

Although in the above-described embodiments, the video signal Vid-inspecifies the gray-scale level of a pixel, the video signal Vid-in maydirectly specify the voltage to be applied to the liquid crystalelement. When the video signal Vid-in specifies the voltage to beapplied to the liquid crystal element, a configuration may be made inwhich a boundary is determined by the applied voltage to be specified,and a voltage is corrected.

In the embodiments, the liquid crystal element 120 is not limited to atransmissive type, and may be a reflective type.

Modification 5

Next, a projection display device (projector) in which the liquidcrystal panel 100 is used as a light valve will be described as anexample of an electronic apparatus using the liquid crystal display ofeach embodiment described above. FIG. 34 is a plan view showing theconfiguration of the projector.

As shown in FIG. 34, a lamp unit 2102 including a white light source,such as a halogen lamp, is provided inside the projector 2100.Projection light emitted from the lamp unit 2102 is separated into threeprimary colors of R (red), G (green), and B (blue) through three mirrors2106 and two dichroic mirrors 2108 arranged inside the projector, andseparated light is guided to light valves 100R, 100G, and 100Bcorresponding to the respective primary colors. Since B light has anoptical path longer than those of R light and G light, B light is guidedthrough a relay lens system 2121 including an incident lens 2122, arelay lens 2123, and an exit lens 2124 so as to prevent optical loss.

In the projector 2100, three liquid crystal displays each including theliquid crystal panel 100 are provided to correspond to the respective R,G, and B colors. The configuration of each of the light valves 100R,100G, and 100B is the same as that of the liquid crystal panel 100.Video signals that specify gray-scale levels of primary color componentsof the respective R, G, and B colors are supplied from an externalhigher-level circuit to drive the light valves 100R, 100G, and 100B.

Light modulated by the light valves 100R, 100G, and 100B is incident ona dichroic prism 2112 in three directions. In the dichroic prism 2112, Rlight and B light are refracted at 90 degrees, while G light goesstraight. Accordingly, images of respective primary colors are combined,and then a color image is projected onto a screen 2120 by a projectionlens 2114.

Since light corresponding to the respective primary colors of R, G, andB is incident on the light valves 100R, 100G, and 100B by the dichroicmirrors 2108, there is no need to provide color filters. Transmissionimages of the light valves 100R and 100B are reflected by the dichroicprism 2112 and then projected, while a transmission image of the lightvalve 100G is projected directly. Therefore, the horizontal scanningdirection by the light valves 100R and 100B is opposite to thehorizontal scanning direction by the light valve 100G to display amirror image.

Examples of electronic apparatuses include, in addition to the projectordescribed with reference to FIG. 34, a television, a view-finder ormonitor-direct-view video tape recorder, a car navigation system, apager, an electronic organizer, an electronic calculator, a wordprocessor, a work station, a video phone, a POS terminal, a digitalstill camera, a mobile phone, and an apparatus including a touch panel,and the like. The liquid crystal display is of course applicable tovarious electronic apparatuses.

REFERENCE SIGNS LIST

1: liquid crystal display, 30: video processing circuit, 100: liquidcrystal panel, 100 a: element substrate, 100 b: counter substrate, 105:liquid crystal, 108: common electrode, 118: pixel electrode, 120: liquidcrystal element, 302: delay circuit, 304: boundary detection unit, 3041:first boundary detection unit, 3042: second boundary detection unit,3043: determination unit, 306: correction unit, 308: D/A converter, 312:boundary detection unit, 3121: current frame detection unit, 3122: delaycircuit, 3123: previous frame detection unit, 3124: determination unit,2100: projector.

The invention claimed is:
 1. A video processing method which corrects avideo signal, the method comprising: specifying a voltage to be appliedto a liquid crystal element for each pixel and defines the voltage to beapplied to the liquid crystal element on the basis of the correctedvideo signal, the method comprising: a risk boundary detection step ofdetecting a risk boundary which is a portion of the boundary between afirst pixel whose applied voltage specified by the video signal fallsbelow a first voltage and a second pixel whose applied voltage exceeds asecond voltage higher than the first voltage, and is determined by atilt azimuth of the liquid crystal; and a correction step of correctingthe video signal, in at least one field of a plurality of fieldsconstituting one frame so that a period in which the risk boundary ispresent in one frame period is shortened, wherein in one field of oneframe the video signal is corrected, whereas in other field of the oneframe the video signal is not corrected.
 2. The method according toclaim 1, wherein, in the correction step, a video signal which specifiesa voltage to be applied to a liquid crystal element corresponding to thefirst pixel adjacent to the risk boundary detected in the risk boundarydetection step or liquid crystal elements corresponding to r (where r isan integer of 2 or more) continuous first pixels on an opposite side ofthe risk boundary from the first pixel is corrected to a video signalwhich specifies the first voltage or higher in any field.
 3. The methodaccording to claim 2, wherein, in the correction step, a video signalcorresponding to the first pixel as a correction target is corrected toa video signal of the maximum gray-scale level.
 4. The method accordingto claim 1, wherein, in the correction step, a video signal whichspecifies a voltage to be applied to a liquid crystal elementcorresponding to the second pixel adjacent to the risk boundary detectedin the risk boundary detection step or liquid crystal elementscorresponding to s (where s is an integer of 2 or more) continuoussecond pixels on an opposite side of the risk boundary from the secondpixel is corrected to a video signal which specifies the second voltageor lower in any field.
 5. The method according to claim 4, wherein, inthe correction step, a video signal corresponding to the second pixel asa correction target is corrected to a video signal of the minimumgray-scale level.
 6. The method according to claim 1, wherein, in thecorrection step, a video signal which specifies a voltage to be appliedto a liquid crystal element corresponding to the first pixel adjacent tothe risk boundary detected in the risk boundary detection step or liquidcrystal elements corresponding to r (where r is an integer of 2 or more)continuous first pixels on an opposite side of the risk boundary fromthe first pixel is corrected to a video signal which specifies the firstvoltage or higher in any field, and a video signal which specifies avoltage to be applied to a liquid crystal element corresponding to thesecond pixel adjacent to the detected risk boundary or liquid crystalelements corresponding to s (where s is an integer of 2 or more)continuous second pixels on an opposite side of the risk boundary fromthe second pixel is corrected to a video signal which specifies thesecond voltage or lower in the field.
 7. The method according to claim1, further comprising: a movement detection step of detecting aboundary, which changes from a previous frame one frame before a currentframe to the current frame, from among the boundaries between the firstpixel and the second pixel, wherein, in the correction step, the videosignal corresponding to a correction-target pixel determined by the riskboundary detected in the risk boundary detection step in the boundarydetected in the movement detection step is corrected.
 8. The methodaccording to claim 7, wherein, in the correction step, the video signalcorresponding to a correction-target pixel determined by the riskboundary moved pixel by pixel from a previous frame to a current framein the boundary detected in the movement detection step is corrected. 9.The method according to claim 1, wherein, in the correction step, thevideo signal corresponding to a correction-target pixel is not correctedin any field of a plurality of fields.
 10. The method according to claim1, wherein, in the correction step, the video signal corresponding to acorrection-target pixel is corrected for each of a plurality of fields.11. A video processing circuit which corrects a video signal specifyinga voltage to be applied to a liquid crystal element for each pixel anddefines the voltage to be applied to the liquid crystal element on thebasis of the corrected video signal, the video processing circuitcomprising: a risk boundary detection unit which detects a risk boundarywhich is a portion of the boundary between a first pixel whose appliedvoltage specified by the video signal falls below a first voltage and asecond pixel whose applied voltage exceeds a second voltage higher thanthe first voltage, and is determined by a tilt azimuth of the liquidcrystal; and a correction unit which corrects the video signal in atleast one field of a plurality of fields constituting one frame so thata period in which the risk boundary is present in one frame period isshortened, wherein in one field of one frame, the video signal iscorrected, whereas in other field of the one frame the video signal isnot corrected.
 12. A liquid crystal display comprising: a liquid crystalpanel which has liquid crystal elements with liquid crystal interposedbetween pixel electrodes provided to correspond to a plurality of pixelsin a first substrate and a common electrode provided in a secondsubstrate; a video processing circuit which corrects a video signalspecifying a voltage to be applied to a liquid crystal element for eachpixel and defines the voltage to be applied to the liquid crystalelement on the basis of the corrected video signal, wherein the videoprocessing circuit includes a risk boundary detection unit which detectsa risk boundary which is a portion of the boundary between a first pixelwhose applied voltage specified by the video signal falls below a firstvoltage and a second pixel whose applied voltage exceeds a secondvoltage higher than the first voltage, and is determined by a tiltazimuth of the liquid crystal, and a correction unit which corrects thevideo signal in at least one field of a plurality of fields constitutingone frame so that a period in which the risk boundary is present in oneframe period is shortened, wherein in one field of one frame the videosignal is corrected, whereas in other field of the one frame the videosignal is not corrected.
 13. An electronic apparatus comprising: theliquid crystal display according to claim 12.